I recommend batteryspace.com as well.
One thing I've found is that it can be somewhat difficult to find a distributor that will sell you legitimate, name brand cells without trying to sneak in a Chinese knockoff under the radar. You need to be very careful about that, not only can the capacity be off, but the protection circuit can be either badly designed, under-performing, or completely nonexistent, which can be dangerous. Amazon/eBay are both horrible about this.