I might be wrong but I don't think there are many TKL boards available. They are harder to find, maybe someone else here could help. In any case, if you want, you could buy a pre-built TKL or go for a similar layout, like a 75% or 65% (basically a 60% with arrow keys and a couple navigation keys.

In any case, these are some good sites to start looking for parts (just the ones I know, I'm no expert either):

https://kbdfans.com (has some nice complete kits, great to start and not worry much about parts and compability)

https://www.1upkeyboards.com (lots of kits and parts, there is a TKL section but when I try to enter it says "No products were found matching your selection", but I just think there isn't any available atm)

https://kprepublic.com (has more variety and a better price range)

https://mechanicalkeyboards.com (good variety of pre-builts)

As I said I'm no expert in building customs, but if you have any other questions I can try direct what you should be looking for/researching. :)