SearchCostaRica has a new web address:
http://www.shuttlebus.co.cr/ (same service which allows you to pay through PayPal and does not compel you to fax a credit card number to InterBus; the service DOES use InterBus for the transportation aspect)
Interbus remains the same at
http://www.interbusonline.com/ but has a new look and "feel". From my experience, it's on the fritz a lot, although the service has not missed yet.
I use
http://www.shuttlebus.co.cr/ because of its ease; the fact one can pay via PayPal and the fact it's a small, one person operation. The owner was the primary programmer for InterBus; has friendly and familial relations with InterBus, but moved to the USA and set up his own business. The airport transfers are always less costly as are the other trips you can book. Not nearly the same discount it started out with....but a dime is a dime is a dime.