Try www.adbility.com for that sort of information. It's not the same as when ad guru Mark Welch was running it but it's still the best bet for finding a suitable ad program.
You may also want to use a program like Commission Junction (www.cj.com) and take the categories of ads you want from their lists of categories, then put all the ad codes into a database and have your own code rotate them throughout the webmail.
Is Canada the best? Noo Doot Aboot It!
"And on the eighth...day...God...created...Mankind. WHY WAS HE HAVING SUCH A BAD DAY?" -- Mick Foley