@ zeuce: It's the exact same kit as the one they use on the FX. I got mine from Lanz Shooting Supplies, but at the time it was the only kit they had left so I'm not sure if they'll have any in stock. I did however get replies from Wanstalls and Reliable Gun saying they have them or will have them in stock by this time. So try giving them a shout.
In my opinion, for all intents and purposes, the FX is nothing more than the SU16C with all the overpriced Kel Tec goodies and an EOTech slapped on. The reason most people up until now have been making their own mods to the original lineup of SU16's is because Kel Tec simply cannot manufacture their own upgrades fast enough for the market (i.e. the AR stock adapter kit and the handguard with bottom rail). Check their website and they'll have little to nothing in stock. That reason and the fact that you can do your own build for much cheaper than the FX. All depends on what you want to spend really. There are some very interesting user made mods coming from the KTOG forums involving a selector switch as opposed to that push-button safety (which gets a lot of gripes) and an AR style bolt-catch/release. Would be nice to see these types of mods become a part of Kel Tec's accessories not too far down the line.
And contrary to what some have been saying on this and other forums, the SU16 family is a very popular range of .223/5.56 rifles, especially in the states. And because they don't commonly come with the AR style stock, the anti-gun hippies haven't focused on it much so there's non of that "Ooooo it's an assault rifle, ban it!" nonsense. Also, any of the issues the polymer receiver may have had back when the early models came out have been resolved, so don't listen to the "me gun went splodey!" crowd when it comes to deciding to purchasing the SU16F (or FX if money isn't an issue).
Best advice I can give, do the research, see what suites your needs/tastes and budget and roll from there. I don't regret doing the build myself.