I have an ipad version 9.3.5(13G36) and have mistakenly deleteed the keynote app. The curerent app is not downloadable to my ipad. Is there anything I can do other buy a new ipad?

Apple no longer allows downloading of their older apps for their older devices.

This started when Apple made the 64-bit hardware/software switch with iOS 11, back in 2015/2016.

If you delete any of Apple's older versions of their installed apps, there is no longer any way to retrieve these older versions, any longer.

They are gone forever.

Also, FYI,

You should be made aware that these old iPad 2, 3, 4 and 1st gen iPad Mini models are nearing 8 and 9-years old, now.

As of the beginning of 2019, many, MANY third party app developers are all more quickly and completely dropping older app support for all old 7, 8 and 9-year iPads that are running 2-3 year old and earlier iOS versions.

Third party app developers are removing all of their older, legacy versions of their apps from Apple's App Store servers, now.

Older versions of apps are going to be gone for good, now!

Getting older versions of app for these old iPad models, initially from Apple’s iOS App Store is getting to be very difficult and problematic, now, and will only be getting much worse in 2020.

These old iPads are now reaching their complete end of life and support.

Old iPads that can no longer be upgraded/updated will have NO MORE APPS AVAILABLE for initial download, at all, soon, from the Apple iOS App Store, for such old iOS devices in the very near future.

Older versions of apps for older iPads will be completely gone, very soon!

This is now the time to seriously consider a new or discounted, refurbished iPad from Apple or other Apple authorised resellers.

Best of Luck to You!

Whilst the App itself, for iOS9.x, has been withdrawn from the App Store, there may be one way to recover the App to your iPad. Please note, I’m unable to verify that this will work for you as I don’t have access to any devices that old - but if otherwise abandoning hope, you have nothing to lose in trying...

First, backup your iPad using iCloud or iTunes - preferably both.

Now, follow the processes to complete a full system restore - using iTunes on your PC (or Finder if using a Mac). This will reinstall a “full” copy of the iOS image for your iPad - that should include all “standard” Apps. Follow this process - you can ignore the first paragraph:

https://support.apple.com/en-us/HT201263

I suggest that for the initial restoration that you don’t attempt to restore from your backup. Just complete a “bare” system restore to verify that the standard Apps are reinstalled along with the OS. Once verified, one way or the other, you can restore your iCloud/iTunes backup to your iPad:

https://support.apple.com/en-us/HT204184

Best of luck. I hope this works out for you - either way, you’ve nothing to lose but a little time in finding out!

Amazon, itself, currently, is selling, new sealed in box, Apple's base model 2019, 7th gen, 10.2 inch screen, WiFi ONLY 32 and 128 GBs iPad models selling for $249 and $329, respectively.

https://www.amazon.com/gp/product/B07XQYPM2N?pf_rd_p=ab873d20-a0ca-439b-ac45-cd78f07a84d8&pf_rd_r=6HW54HFD0YSC2ZE3CP7Z

The 2019, 7th gen, 128 GBs iPad model is the only one still, available, currently, from Amazon's own direct sales distribution.

These 2019, 7th gen iPads have much faster processors, at least 3 times the internal RAM, iPadOS 13 support, Apple Pencil and Apple Smart keyboard cover support and have better audio speakers and built-in cameras.

A lot has happened with iPad and iOS in 7-9 years and in the past 2 to 3-1/2 years since iOS 9 and 10.

Best of Luck to You!

Log into the App store. Go to "Purchased," find Keynote, and see if that allows you to install the older iOS 9-compatible version.