This. Sometimes you can find a manufacturer or model on the valve by using your smartphone camera. Try several angles. Take those pictures with you to the stores.

Pex is relatively new so you have a good chance to find that valve still being sold.

Clean the valve seat (in the valve body) gently when you are reassembling. A small amount of faucet grease on the seal is a good idea but not required.

BTW: to avoid breaking a valve, when you start to turn, if it resists; turn just a little CCW, then back CW to tight, then back CCW a little more... Back and forth can break the corrosion up without breaking the stem. It usually works.