There are a variety of summer sublets available usually. Either from students leaving New Haven for the summer, or students moving away permanently and needing someone to finish their lease. Yale Station and Craigslist are two useful resources for locating sublets. The best source of summer sublet information is the multitude of flyers advertising summer housing that populate the bulletin boards around campus in the early spring.
How to not eat Ramen noodles all summer: Financing your Research