you saw the site, now what does it say?
It clearly says you can book online, they'll get you a voucher, and with that you can make your actual reservation with the
testing center.
Most of those you will be able to contact online through
http://www.prometric.com but some might require a phonecall I guess.
This is all information you can get from the Sun certification site, I don't see how you could have visited the site and not found that information...