Wesleyan is an LAC that offers graduate courses through the doctoral level in Math. As you can see from below, it leans heavily toward Topology which is about as “pure math” as you can get. As a transfer student (Wesleyan accepts about 50 a year) you might be especially interested in a combined BA/MA in Computer Science which only requires six one semester courses at the graduate level:
http://www.wesleyan.edu/mathcs/graduate/regular_courses.html