The mags are a little pricey, but if you shop around you can sometimes find SOME of them at a reasonable price. (CDNN has them, but theirs aren't cheap -- and you have their flat rate shipping charges, too.) (I think I paid $35 each,
shipped for the extra ones I've picked up in both .40 and 9mm. I've paid more than THAT for a number of different factory mags from SIG or CZ, before shipping is added.) I think the FNS9c are just starting to be available from vendors, but may be available from FNH. Hopefully, Mec-Gar will offer mags for the FNS and FNX, one of these days.
Magazines: Try the
High Plains Gun Shop -- but you'll probably have to really flip through a lot of screens to find what you're looking for:
http://highplainsgunshop.com/
Conversion Barrel: Here's the link to EFK Fire Dragon:
http://www.efkfiredragon.com/fnp-and-five-seven.html
Like most barrel makers, if they don't have what you need in stock, you'll have to ask to be put on their waiting list and ask them to notify you when they have a batch available. I waited a couple of months, got an email saying they were available, and called them -- and had the mag about 3 days later.
I haven't found any other firm selling conversion barrels.