I can guarantee at least the following flights from these cities/carriers:
Delta, ATL - daily direct
AA, JFK - daily direct, and connection in SJU
jetBlue, BOS - daily direct, and connection in SJU
I am sure others will come up, like direct from CLT on AA/US, and EWR on UA.
When are you going? If in May, the above direct flights may not have started. Anytime June-August is 100%