I personally use ssls.com. It is slightly cheaper than namecheap.com for multiple year, and it is actually namecheap.com by another name.

On a side note, I would only buy one year certificates at this point, because Lets Encrypt is about to make them free.

I second Namecheap. I use the same SSL certificate for my web service. I used to use free cert from startSSL but they declined to renew the free one and the cost was too high so I switched to Namecheap.