Crown caps for American bottles are all the same size. You can find them at your LHBS, online stores, or specialty places for personalized caps. They normally sell in bulk starting at 100. If you plan to store/cellar for a long time, I suggest the oxygen barrier caps. They are slightly more expensive but worth the peace of mind.
Airflow is not the issue, these caps have a chemical added to the inside liner that actually "absorbs" oxygen from the head space of your beer. Use these on beer that is stored (aged) for a long time, such as barleywines, or for competition purposes.