My clients also used www.vrbo.com - with positive results.
Homeaway.com is very popular.
Flipkey.com is yet another one. Didn't use that one, but got their promotion.
In addition to that, we do have seasonal rentals with private owners - in condotels.
These are advertised on Realtor.com in rentals.
Hope this helps,
Beachfront Realty, Inc.