You've probably seen the same sites I've looked at. HackerRank, CodeForces, Geeks for Geeks, HackerEarth, Sphere Online Judge (SPOJ), DM:OJ. Of those, I've seen paid options (with "email us for pricing" type sales hooks) on HackerRank and for the Sphere Engine used in SPOJ.
Another site (with beginner problems problems in Java and Python) is CodingBat, developed by Nick Parlante while lecturing at Standford U. It's got teacher support and problem authoring support, and is intended for small practice exercises, rather than larger projects. See the "about" and "help" sections at codingbat.com.