Do a search for "pipefish" in the oddballs and brackish sections and you'll get quite a bit of stuff. As Fella says, these are not easy fish. Personally, I'd put them in the category of fishes that are not really suitable for any but the most experienced aquarists. Your problems start with identifying them. If at all possible, try and buy them from stores that use Latin names. This makes your job a 100x times easier, because you can get the water chemistry right. Wildwoods, for example, has Microphis deocata in stock at the moment. These are true freshwater pipefish. Other stores often have subspecies of Microphis brachyurus and these tend to need brackish or marine water.
Your second problem is feeding. Essentially they will only take live food. While they aren't picky about what you use -- bloodworms, daphnia, and brine shrimp are all good -- they are only adapted to frozen alternatives with difficulty. I'd suggest getting a pond or water barrel established in your garden for culturing daphnia and insect larvae, and use these to feed the pipefish.
Adapting these fish to your tank is not easy, and changes in temperature and water chemistry must be avoided.
Once settled in and feeding, these are outstanding fish. Adults can be quite large and when scooting about they are quite hypnotic animals that do repay the extra effort required to keep them. Cologne Zoo has a lovely tank of them, and many zoos have bred them in captivity, maintaining populations for decades. So they aren't intrinsically averse to aquarium life, just demanding.
Cheers, Neale