Most are no different from regular house faucets. Certainly everything in Mobile and modular homes Plumbing is just the same.
It is best top replace the tub facet with the valve that are compatible to avoid leaks or the parts not fitting properly. Home Depot or Lowes should have the parts available.
You generally have to cut a hole in the wall behind it.
The national retailers Lowe's and Home Depot offers claw foot tub faucets for sale. The web sites Amazon, eBay, Vintage Tub, and Faucet Direct also offer these faucets for purchase.
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Rom tub faucets can be found at your local Home Depot. Another well known home improvement store that sell them is Rona. Some lesser known companies that sell Roman faucets are Delta Faucet and Faucet Warehouse.
Replace the tub faucet.
A tub faucet acts as a shower and can be used to bath in the bath tub. It is a pressure valve which makes the flow of water through it constant and gives a nice shower.
Faucet side.
If the faucet contains the re-direction valve (closes off the water from the faucet and redirects the water to the shower head) you need to rotate the faucet counter clock wise to unscrew it from the plumbing and replace it with a new one. Take the old one to your local box store and they can match it up for you.
Most of them are in the wall. The other would be a deck mount, which is mounted to the fiberglass tub deck and uses a Roman Tub Faucet.
It depends on the make of the faucets.