Yes, you can get them.
No, you probably don't want to.
I've seen freshwater pipefish for sale in three shops here in London over the last 12 months. Species of Microphis I believe. Anyway, they are extremely difficult to maintain. The first problem is identifying them: a few species are strictly freshwater, but many of them are estuarine and migrate freely between fresh and salt water. You need to get a Latin name for the species being offered, and then check up on Fishbase. Aquarium books rarely carry any species of them, the exceptions being the high-end catalogues like Baensch's Atlases.
Even assuming you have identified your species, the problem you next need to deal with is feeding. With no exceptions, these animals will only readily take live foods. Possibly you can wean them on to dead foods later on, but to begin with, it's daphnia, bloodworms, and brine shrimps. Some species are benthic (Enneacampus spp.) and eaty wormy foods happily enough, while others are midwater and only feed on swimming things (Microphis spp.).
I tried pipefish once, lost them within 24 hours. By contrast, I kept a seahorse for years. That's how difficult they are. I think I lost mine because it was a juvenile brackish water fish and my aquarium had soft/acid water, so the immediate issue there is water chemistry shock. But it might have been that I transported them from store to home on a very cold day, and they suffered from heat loss.
On the other hand, in a quiet tank with the right water chemistry and copious live foods, they could do well. Freshwater pipefish are regularly maintained in public aquaria, and I have seen some very large and healthy specimens over the years. They look fantastic in the right tank. I have some pictures of an amazing pipefish tank
in Cologne Zoo that is certainly inspirational.
Cheers,
Neale