I don't know of any problems currently in the NPL that are tagged specifically for Thomas' Calculus, but if you have a look in the "Advanced Search" area of the Library browser, then you can search for problems from other books. That might be helpful if you are familiar with another book and how it corresponds to the material in the Thomas Calculus book.
Also, Pearson has just hired a webwork problem author to write problems for one of their Calc 3 textbooks (University Calculus I think), and they did so in response to a request from a school which had adopted that book. Those problems should be available next summer or fall.
So, have a look around the NPL, and I'll bet you find plenty of problems that suit your course, but if you want problems written specifically for Thomas' Calculus (or any other book), my advice would be to nag the publisher about it and maybe they'll hire someone to do it.
In that case, if you or they don't have someone in mind for the job, send them to me and we'll find a problem author to do the work.