There isn't really a "preferred" store or website for getting a CBB. Australian CBB's have a reputation for being more hardy, but you will pay handsomely for the privilege. If it's at a LFS, I'd definitely have them feed it and see what kind of response you get before buying. I personally like LiveAquaria because of the 14 day guarantee. Supposedly fish are being harvested less and less with cyanide, so you can only hope that this is true. The biggest problem I see with new CBB's is that they are very thin.
I've had several CBB's over the years, including a couple that have died in my QT. Whether it was normal stress or bad harvesting techniques I'll never know. I did have one die in my DT because, IMO, and in retrospect, he wasn't getting enough food because he was outcompeted by the other fish. I plan to build a CBB feeder for the next CBB that I buy. And just so you don't think I'm a CBB killer, I had at least one (maybe more, but I can't remember) that lived for years in my DT before I had to break down the tank.
Finally, as noted above, CBB's are worm eaters. Live blackworms are by far the best food for them, and if the CBB won't eat them it generally is a goner. Although I haven't tried them yet, Paul B. suggests live whiteworms as a food. The benefit to them is that they can be raised by you/me and tossed in the tank as needed. I've heard other people use small red wiggler worms with success, and of course, live clams. Paul B. likes to freeze his clams and shave off some pieces for his CBB.
If you want to be skinny, do what skinny people do. If you want to be successful with CBB's, do what Paul B. does.