Check valves for 1/2" or 3/4" Vinyl Tubing?

[Enigma]

Where can I find check valves for 1/2" or 3/4" vinyl tubing?

I think I've got all of our plumbing parts together for our drilled tank and sump. I haven't been able to turn up check valves for our return, however. I'll need either one 3/4" check valve or two 1/2" check valves (our return splits from 3/4" off of the pump into two 1/2" lines).

Any recommendations as to where I can find these?