@turquoise-- If you're looking for a bath tub that's going to benefit your hip, you need a hip bath tub, not a sitz bath tub. Sitz baths are tiny and like the article said, your hip won't even get immersed in water with a sitz bath tub.
A sitz bath tub is like 1/3 the size of a hip bath tub. In the past, sitz bath tubs used to be ceramic or steel. These days they're all plastic and even smaller. They even have ones that fit into the toilet now and you put water in it and just sit on the toilet.
For hip joint pain, I do recommend using a hip bath. It's not terrible affordable, but it's an investment and you can use it for years and years. I've had mine for a long time, I bought it from a company that makes bath tubs and other kitchen stuff. I specially ordered mine because I'm pretty tall and wanted to make sure that I fit in it.
And if you want to immerse your hips and knees and stomach, make sure to look for a deeper tub because they come in various sizes.
To buy one, I recommend finding companies that specialize in bath products and then check their website or call them up to find out if they carry hip bath tubs as well.