I call APC and order them direct. After all, it's not like you're going to order just 1 or 2 props, right?

I don't know of any online hobby shop that sells them.

Tower might, but I'd have to check. I don't think they have the right picture there though. I forget, is there a 9x3 non-combat prop? If not, then any APC 9x3 is going to be ok.