A picture would definitely help, at least a description of whether a single handle like pic in previous post or two handles.
Your shower valve is almost assuredly a regular home-style shower valve. 12v or lower pressure have no bearing on shower valve selection.
I am also confused by description of being "seized.". The guts of the valve is a cartridge that can be removed and replaced. These are typically brass in older units, ceramic or plastic in newer ones; with o-rings. There is nothing to seize (per se) but can be cleaned and lubed. Or the insert cartridge can be replaced On older or obscure valves, you can often take the cartridge to a specialty plumbing store (the old style ones, not a Home Depot).
Random website explaining replacing shower valve cartridge.