1968 C10 Gas Cap

[me71c10]

Can anyone recommend where to buy a replacement gas cap for a 68 C10? I used the "search" tool and realize that the easy answer is to go to local parts store and buy one that is vented. The problem is that it only vents in not out. When I remove the nee cap, air build up releases which never happened with my old cap with a bad gasket. I can't imagine that much pressure build up is a good thing. I've never posted posted before because I usually can find my own answers on this site but this time I've had no luck. Please educate me.

Thanks