The basic o-rings guys use are 013's for everything. The THO store carries them.
If you want something a little larger, check out 014's and 015's.
I also have 906, 907, and 908 in my tackle box kit. They are a little more expensive at times because they are not as common of a size, but they fit right in to call making. I personally use 906 more than any other size.
Check out The Rubber Store and The Oring Store for pricing. Just make SURE no matter who you buy from that you are getting neoprene rings. (Not buna.....they will not hold up well).