how to find a college that offers assistantship programs?

Unfortunately, there's no easy, one-stop place to look. You're probably going to have to look on each department's website, one by one, for all the schools you're considering. Some of them will offer an assistantship to every grad student they admit; some only some of their grad students; some don't offer them at all. Some schools will want a separate assistantship application, some just consider everybody who applies for admission. If it's not easy to find the information on their website, there should be an address there to email for more information on the program--try that.

Just look up colleges you're interested in and go to the website for the department. They'll mention what, if any, support they give to grad students. Assistantships are often reserved for PhD students, I'd be surprised if you found many for master's programs in the humanities.