RockAuto usually carries emissions parts from PCI Incorporated. Their direct website is: PCI Inc.
They list two check valves, one threaded, one without threading. The item numbers are 37 and 39. One of those should be what you are looking. FWIW the model years these are supposed to fit is listed for 1985 through 1996.
PCI Inc. is usually pretty good to deal with. I have called them a few times to verify the parts I needed as well as availability. Great service!