For just a basic SSL level 1 cert, Used to use StartSSL, until they started to decline renewals. Seems they would look at the site and if they found you used a PayPal cart, they would say you had to use a Level2 cert which cost more than a basic level GoDaddy Cert... kind of stupid since the PayPal cart uses PayPal's servers and its certs. They would not budge on this even though the renewal was for a cert they approved the year before.
Now use Namecheap for normal certs (about $9 a year)
https://support.namecheap.com. Unlike StartSSL, you can rekey for no extra fee which is especially nice if you are dealing with someone that is giving you a CSR and doesn't know what they are doing.
If I need a UCC/SAN cert (Multiple domains), I use GoGETSSL
https://gogetssl.com which was around $105/yr (6 domains).
If you are doing anything merchant level though, you need to use a merchant SSL cert which is class 3 or better.