Where to find specific endgame practice exercises/puzzles/practice bots? I'm looking for a set of exercises (contained in a book, website, or chess engine) where I can practice what I know in regards to rook and pawn vs. rook endgames. I could find a friend to randomly place pieces on the board for me, but I don't see what would be in it for her or him.

Something ideal would be a website that displays a puzzle that I can solve, but the pawn would have to be on the a- or h-file and 7th rank. If I can set these parameters while randomizing the position of the rooks or king, that would be great.

Where can I find such a website, and if there isn't any, how can I create such an object? The object would set the pieces randomly on the board, with the exception of my selection. So if I select the pawn to be on a7 and the king to be on a8, it will randomly place the other pieces elsewhere. I'm only interested in the following pieces being on the board: A pair of kings, a pair of rooks, and one pawn.
If you're looking for a truly random position, I'd just roll an eight-sided die a couple of times to determine the column and row of each piece. (If the position is illegal, just reroll.) You can then enter the position into a chess program and play against the engine. Here's an online dice roller in case you don't have physical ones, the one you're looking for is called the d8.

If you're looking to practice key positions in KRP vs. KR specifically, you'd probably be interested in John Nunn's Secrets of Rook Endings. I forgot how this book is organized exactly, but it contains almost all positions in this ending which are worth studying.