My ipad had only 16 GB and run out storage. How can I purchase more storage to iPad.

You don't.

You own a low storage, 16 GBs iPad model.

There is little to be done.

So, with all of this stated,

A 16 GBs storage iPad starts out, after initial drive formatting, at approx. 14.5 GBs of free, internal storage.

After the installation of iPadOS, at approx. 6.5 GBs, leaves 8.0 GBs of of free, internal data storage space.

iOS/iPadOS needs a minimum of 3-6 GBs of constant remaining storage, that MUST BE MAINTAINED AT ALL TIMES, to insure/ensure nominal, acceptable operation of all iOS/iPadOS functionalities.

This leaves a 16 GBs iPad with ONLY a practical, usable storage space of between 2 and 5 GBs AND THAT IS IT!

This is virtually NO internal data storage to do much of anything with that iPad!

This is why Apple quietly, silently did away with all 16 GBs iOS device back sometime in 2016.

A 32 GBs storage iPad starts out with approx 30 GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves between 23-24 GBs.

Minus 3-6GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 18-20 GBs of actual, usable data storage space.

Even with 20 GBs of actual, usable internal data storage space in an Apple iOS/iPadOS device,

a 32 GBs data storage devices are, now, more quickly, becoming the “new” low storage 16 GBs internal data storage space devices, today, a 32 GBs Apple iOS/iPadOS device has 3-4 times the internal data storage space of older base model, low internal data storage,16 GBs. internal data storage Apple iOS/iPadOS devices.

In reality and in use, in 2021, a 32 GBs internal data storage device is kind of a joke and of limited usefulness, when up to 2TB of internal data storage ( approx. 1.8 TBs or usable internal data storage ) portable tablet devices exist today.

Apple just put an end to 32 GBs devices, in September, with the base model iPad now offering 64 GBs of internal data storage ( approx. 50 GBs of actual, usable internal data storage ), which, for a few more years, anyways, has enough storage to make an iPad useful for longer period of time before data storage fills up.

Purchasing a low storage Apple device, to save on the cost of the device, is a false economy.

You really need to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions, with much larger internal data storage ootions.

There are PLENTY of less expensive, newer, older iPad models that can run the latest, current versions of iPadOS.

If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2014 and later.

All of these older model iPads below, CAN run the latest releases of iPadOS 15.

2014, 9,7 inch screen iPad Air 2 models ( that model came in 64 or 128 GBs internal data storage configurations )

2015, 7.9 inch screen iPad Mini 4 models

2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )

2016, 9.7 inch screen iPad Pro models ( 1st gen Pencil support )

2017,10.5 inch screen iPad iPad Pro models ( 1st gen Pencil support )

2017, 9.7 inch screen iPad 5th gen models

2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )

2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )

2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )

Get the largest internal data storage model you can afford as NO Apple iOS device has ANY expandable storage.

I am sorry that you had to learn all of this, but this does not change any of the facts regarding these very old iOS devices.

Sorry & Best of Luck to You!

If you decide to go down the path of purchasing used, secondhand Apple devices, here is how to avoid purchasing an Apple iOS/iPadOS device with user activation lock, in the future.

If you purchase any iPad, or any Apple iOS device, for that matter, used/second hand, online, or even in-person, make sure this iPad does NOT have user activation lock and has been properly reset for sale.

A properly reset for sale iPad should initially power up to an animated, multi-language “Welcome” screen.

DO NOT take the iPad seller's word that this iPad is user activation lock free!

We have seen plenty of used iOS device purchasers get scammed.

User Activation Lock can never be bypasss, circumvented or worked around IF there is no correct previous/original user information to be able to do so.

Apple doesn't get involved with ANY user activation locked, used/second-hand sold devices, either.

So, buyer beware!

If purchasing a used device in-person, power the iPad ON

If the iPad DOES NOT display the initial animated, multi-language “Welcome” screen, then pass on purchasing this iPad.

If the iPad powers up to any other screen, then forget purchasing this iPad.

You cannot trust that the seller actually has the correct needed info to properly reset the iPad back to factory settings right then and there before the actual purchase and exchange of money for the iPad, in question.

Pass on the device!

If you are purchasing this used iPad online ask the seller to provide multiple images, either online or be sent to you via email, showing this animated, multi-language “Welcome” screen of the interested in purchasing iPad model.

Even better, ask for a short video be sent to you showing this animated, multi-language “Welcome” screen, in action and in its entirety, posted to the online ad or emailed to you.

If the online iPad seller refuses any of these requests, pass on the iPad. DO NOT buy!

Good Luck to You!

You cannot add space to an iPad, the space is fixed as purchased. If you need more space, you need to purchase a device that comes with most space to begin with.