We have the teapot bathroom faucet and need to replace the hot water valve - where can we purchase this part and how much is it?

There should be a brand name printed somewhere on the body of the faucet. Google that name and call manufacturer. They can either sell you that part ( or even better, send you one for free ), or they will direct you to their distributor in your area.

If it is a common faucet, you should be able to get replacement parts in your local plumbing store.

If there is no brand name visible, see if you can take photo of it and post it here. Perhaps, one of us will be able to identify it for you

Hope that helps

Milo

Is it the valve under the sink or the cartridge in the faucet?