I have never seen a rhizo for sale locally in over 4 yrs in the hobby. I don't know where rhizos are in the wild, but pretty sure they're still illegal to collect in some areas. If you find one on the market, it's probably illegally exported or a healthy specimen that's been around a while (hopefully the latter). I've really only seen a few pop up online.
FAOIS has one in the 90 gal display tank. Coral Corral had on when they had the non-photosynthetic tank when they had it running.
Rhizos are a really nice coral in person. For the op, I don't know if you've ever heard of a knuckle coral, but it may be a nice substitute? Granted, it's not as large as a rhizo, but they're pretty interesting if you read up on them. They're pretty rare to find also but they're available time to time.