Where can one purchase a tub chair?

Go to your local pharmacy (such as Walgreen's or even supermarket pharmacies), where they sell canes, crutches, ect. You will also see items such as walker accessories, raised toilet seats, and shower chairs. Get one that has a seat back so you can get some good support. You can also look into getting a doctor to prescribe one for you, and getting one through a company that sells durable medical equipment (DME). If your insurance covers it, that would be a route to go.