You can purchase an SSL certificate with many companies. For the most part they all work on the same number of browsers, the difference tends to be with any warranty or protection they give you, along with what icons or verification seals you are allowed to use. I generally use instantssl.com myself as they have a nice array of products, some which are very cheap, some which come with more and are not so cheap. Geotrust which rackshack and godaddy both offer (I believe) is good as well.