Hours are really a hospital issue. You can certainly discuss extra hours during any hospital interview, but on standard contracts, the goal of the hospital is to limit all traveler overtime hours to the max. That said, some regular contracts do have OT available, and some contracts have very low bill rates for OT (like regular rate plus $10 an hour - this still allows agencies to offer time and a half without losing money) so the hospital does not try to limit needed OT.
But your best bet is to work with a rapid response specialty agency such as Fastaff, CRU48, and On Assignment that offers 48 hour contracts with as many extra shifts worked as you can tolerate. But you will work hard on most of these assignments, they are difficult or troubled hospitals for the most part.