Know this is old; but, may still be relevant.

Try Gotham Travstaff-seems to have the best reputation for Manhattan. Onward Healthcare seems to have a significant presence. I also have met a few that are with White Glove; but, when I tried to call them for some per diem work while on a contract with another company (not all all happy with them so I am not going to mention the name), I got the run around.

Word of warning if considering NYC-make sure you get the ratios and floor training period that is promised to you in writing (ideally, into the contract). Put my license on the line every shift because travelers were dumped on (had 1:12 in the ER) & the are frequently the scapegoat for internal problems for many of the hospitals there that rely heavily on travelers. Beware if the unit has more than 4 travelers or more than one day position open-that's your red flag for problems.