Does anyone know where I can find a shower transfer valve cartridge for my shower panel, I do not know the make of my panel and I'm having a hard time finding the piece I need?

You are not stating its make, type, brand and / or manufacturer. Without this vital information, nobody will be able to help you.

Best is to note the brand and manufacturer. Sticker or brand name should be somewhere on the unit itself. Google this information and look up Contact or Customer Support information.