Inline check valves? I stick with plastic ones. I got some stainless steel ones from TPT seller before. Great guy but they all failed. All 3. This is why I run 3 inline in case. My plastic ones I get from the store for i dunno $2.99? Never had one fail yet. The high quality ones are made by like swagelok or clippard or some industrial gas company. Expect to pay a lot more for these but yes, they are great. But cmon, for the availability and price of plastic? Run at least 2 inline. Mine look like these:
Not those disk ones, but those probably will work as well.