Can’t find right radiator cap?

This is my first time posting here so idk if this is where I’m supposed to be asking questions sorry.. Anyway I recently bought a 2001 Toyota Tacoma pre-runner that was converted to 4x4. I can’t seem to find the right radiator cap to fit my truck.. I purchased one from an auto shop that was said to fit my model, but the spring under the cap is to long. There is a radiator cap already there that fits perfectly, but doesn’t have the spring. I want to replace the previous cap because it seems faulty.. I’ve heard that you need a specific cap in order to maintain a specific pressure so I don’t wanna get the wrong one.. they both say 0.9 on the top, but I’m not sure why they’re different sizes.. Any suggestions?