Write one! Forths are so simple that you can write one in minutes if you understand it, and a bit more if you don't know anything about it. Having your own forth is both really gratifying, and gives you an opportunity to play around with the internals and change things up to see what happens (and why the choices made were made).

Forth is one of the languages where you have to know the internals to write it, but the internals are really simple.