The ones I have come from Granger. I have both polysulfone and nylon, most of what I have are 1/4 inch. This works well on a variety of tubing, if I am using one on a cork I slip 1/4 ID silicone tubing over as an adapter. Granger also had 3/8.
How they work? Typical construction is a stainless ball that hits a vitron sealing surface. There is a small spring which presses the ball and determines the cracking pressure. For wine I have the lowest cracking pressure I can get. Theoretically the spring and sealing surface could gum up, I flush with water and then distilled.