Working in radio I can tell you that tape is now the least desireable format. CD's are now the standard. However, we are getting more and more CD-recordable format PSAs these days, especially from the do-it-yourself PSA distributors (ie. non Ad Council, Goodwill Communications, etc.). These are perfectly fine as well, and are more affordable than doing a commercial run.
If you want to shop around for pressing companies, just do a Google search for "cd pressing" and you'll find a ton of options.
Within 10 years, I suspect a lot of PSAs will be distributed via file transfer - many distributors are querying radio as to their capacity to handle file distribution. As more stations move to fully digital studio operations, this should become an even more cost effective route.
Make sure you include postage paid reply cards if you want stations to report usage back to you. They won't put a stamp on it, and keep it short and sweet. Anything that doesn't require a circle or two is going to the round file. It's even better if you can stick a label on the card with the station info/address already on it. It saves a ton of time, and the more time you save the traffic manager, the better the chance you'll get your card back. Don't ask for post-usage details, ask for the station's planned usage. Many will do at least 10 week runs and when that's over, they are not going to dig around looking for your postcard to report details.
Good luck with your campaign!