This app was mentioned in 1 comment, with an average of 1.00 upvote
I don't know of a free source, but this is very good.
It's by the same company that does CT ART, and they have response lines in addition to being able to play against an engine. If you don't have an Android device I believe they make a similar product for PC.
Usually practicing endgames against an engine is not ideal, for two reasons:
1. Engines will play the same move most of the time, and if there are multiple plans as there usually are you only get to practice one of them.
2. If an engine has a lost position it will often play some random garbage instead of trying the best human defense, which is really what you should be practicing defeating.