Where do I locate my 28 character recovery key?

The 28 character key has nothing to do with unlocking a used iPad. It is used to help you recover an an account.

The information below is based on Apple's support article: Set up a recovery key for your Apple ID - Apple Support with quoted excerpts. Each section outlines the different situations and the strategies that may relate to use of a recovery key. If you are not familiar with trusted devices and numbers, see this support article —> Two-factor authentication for Apple ID - Apple Support

- If you still have access to your Apple ID and password but have lost your recovery key:

"You can generate a new recovery key from Settings or System Preferences on a trusted device." Refer to the steps in Set up a recovery key for your Apple ID - Apple Support about how to do this on different types of equipment (Apple equipment only).

- If you have forgotten your Apple ID password, have lost your recovery key, but still have access to a trusted device.

"you can try to regain access using your trusted device protected by a passcode." The process is outlined in this support article: If you forgot your Apple ID password - Apple Support This does not involve using a recovery key.

- If you have a forgotten password and you do not have access to a trusted device:

"you can use your recovery key, a trusted phone number, and an Apple device to reset your password."

- If you have lost your recovery key and have lost access to both your account and trusted items:

A recovery key is designed as an alternative to Account Recovery ( How to use account recovery when you can’t reset your Apple ID password - Apple Support ) which is where Apple helps you regain access to your account. However, "when you generate a recovery key, you can't use account recovery." You can try to contact Apple (see: Contact Apple for support and service - Apple Support ), but to me the previous statement implies that setting up a recovery key excludes Apple from being able to do anything. Apple says: "Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently."

If the iPad disabled or Activation Locked, unless you can independently obtain assistance from the previous owner, the iPad is (and will remain) useless to you. If you are able to contact the original owner, they will require this information:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as the original sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

The All Too Common SAD Reality of Buying … - Apple Community

Here is what Apple suggests when purchasing a previously used device:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.