I've decided I'd like to get some more practice at endgames. One thing I though might be helpful was to play some "won" endgames v a computer player at various levels. However, I need 2 things to do that, that I don't have.

1. A place I can find a collection of end game FENs (or PGNs, or at worse just positions I can see).

2. A computer program where I can easily enter in a FEN, PGN, or worse case manually set up the pieces and play from a given position. Ideally for an Android phone, but I'd settle for a PC. (Most programs I've seen either don't let me set up a position or are engines that insist on showing me the best moves for both sides instead of just letting me play.)

Any suggestions?

Any endgame book will contain a large number of positions you should be able to win or draw. You can google "play chess online against computer" for sites where you can do that.

1. FEN Source for Endgame Position: Endgame Position FEN List  Practical Chess Endings Positions you can also try this online endgame practising source  

Chess Endgame Simulations
2. Computer Program: Scid vs Pc

For 2, you can go to Learn/Analysis/ in chess.com and load a FEN game. Then, scroll down and click the computer icon to play against the engine.

When I pop in anything from this list... it tells me they are not valid?  Am I missing something?

chesstempo offers an endgame trainer. 

You can find 3985 FENs in my git-repository https://github.com/zabuzara/Chess-Fen/blob/main/FENs.txt for Checkmate and Draw ending with Stockfish.

It's possible, that you find absurd Positions, but I checked alls with Stockfish.

What does it help us that SF checked the FEN when it turns out to be absurd? Like when there is a triple check in the diagram?