Others may be interested, but in the interest of honesty, I wouldn't use this 'method' unless you're completely lost: I'm disabled and this was the only way I could find it on my own:
I went here:
https://stash.reaper.fm/v/8070/ZD-complete.reascale (Here are more options:
https://stash.reaper.fm/u/zappadave)
I clicked 'download' and saved it where I could find it.
I opened Reaper.
I opened the MIDI editor.
I checked 'Key Snap' (Bottom Middle).
Click on 'scale' and scroll to the bottom and find 'Load'.
Browse for AppData/Reaper/Roaming/Data. (If you can't find it--like I couldn't find it--go to (and open): 'Options/Resource Path' in Reaper. Copy location in the Windows 'File Explorer' header: Paste location...go to location...copy 'scales file', and Paste inside the Reaper/Data folder.
Compose.