The reason you don't see them too often is there are very mixed results when attempting to grow them submerged. I wouldn't recommend them unless you have high light, good steady CO2 injection, and good success with other finicky plants. Some folks go so far as to claim many of the more common fissendens species are not aquatic, but I don't believe this to be the case. I had limited success with them several years ago, but never anything to write home about.
eBay is usually the only place I see them - they usually ship out of China or Malaysia.