What size is a #7 O-ring?

I am working on a project and it calls for some #7 O rings. My problem is I have no idea what size that really is. When I do web searches, I can find the ID and OD but I don't see them being sold as a "#". I'm hoping someone more mechanically inclined than I can help give me some guidance on this. I'd also like to know where I can purchase some. Thanks in advance for your help.

lily