I'm having problems with a vacuum in my gas tank that's not allowing fuel to the carb. When I open my gas cap, its sucking in air. Does anyone know where to get vented gas caps for '72 Cuda 340? I found one place online, but shipping is more than the gas cap shocked

Thanks.

1972 Cuda 340 4-speed
2015 Challenger SRT 392
2015 Jeep Wrangler Unlimited Willy's