Where can I find a version of Pages compatible with Catalina

If you have installed Pages previously, or have removed it from Catalina, you can get it back. If you have never installed Pages before, then you cannot get it now for Catalina as the current version (v12.0) requires Big Sur or later, and you would have to settle for Pages for iCloud, iOS, or iPadOS with compatible devices for iOS 15.*.

Launch the Mac App Store and be signed in with the same Apple ID as you used to install Pages previously. Press cmd+0 (zero) to open the Purchased panel, and if you see Pages, it will have a blue outline cloud symbol with a down arrow. Click that, and a dialog will appear offering to install the last compatible version (11.1) of Pages for Catalina.

More information about Pages on its support page: Pages - Official Apple Support

You are welcome.