I am looking for a replacement trim valve kit and I am having trouble finding the right one that will fit my current pressure valve. It appears to me to be a model from American Standard.
Click photo for full size
Shut of the hot and cold supply to the shower. Turn the valve to verify that the shut off are stopping the water. Remove the screws that retain the valve. Bring it with you to your local home center or hardware store. Most stores that I have been in have a catalog that allows you to match your part to pictures in the book.