Well, honestly (and factually), ad-hoc shouldn't be a category by itself. The term "ad-hoc" just means a solution made for a very specific problem (or a problem which requires such solution), and thus it does not serve as a category. Like the word's definition, every ad-hoc problem has a different solution. I've seen "ad-hoc" being used analogous to "idea-heavy" very often, and I think this is a misuse of the term.

i think codeforces problemset is best for ad-hoc.

I think you can find some interesting problems in https://www.spoj.com/problems/tag/ad-hoc-1.

Of course codeforces has wonderful problems too (like always).