Don't know of any web sites but many bars etc. have signs in the window wanting staff. Could be a bit far into the season for this year though.

Having a place in Rhodes, I know many British people that come to the same seasonal bar jobs year after year and just love it. The money is not great, but they get to be considered "locals" and don't pay the prices on the menu.

In order to secure a position for the season, you must find something before May 1st - after that the jobs are rather scarce.