RapidSSL and PositiveSSL should be fine (personal preference is buying through Namecheap) and if you buy the www.example.com cert it will support example.com as well.
You may want to confirm that last bit with wherever you buy the cert--Namecheap does this but I'm not sure if its universal.
You may want to confirm that last bit with wherever you buy the cert--Namecheap does this but I'm not sure if its universal.