You need to buy a sims kit for you specific gun. All simunition guns fire a 9mm sized (slightly different so it won't fire anything live,) round. You buy the kit, replace the barrel and so forth and you can fire sims.
Actually, pistol kits require only a switch of the barrel; you use the factory slide, guide rod, and recoil spring. For an m16/ar kit, you have to replace the entire upper, as it uses the same 9mm based pistol style cartridge.
There's no good reason to buy a conversion kit... The company that makes simunitions sells only to certified military, LE, and a few civilian professional trainers. If you take a class from any of the latter, they'll provide the equipment.
The world being what it is, I'm sure simunitions pop up now and then on auction sites, in the hands of sellers who don't care who the buyer is. That said, messing around with them on one's own isn't the best idea, IMO. Do a search here on "simunition" -- and read this thread, and especially Pax's post -- and this thread, as well, to get some idea of why.
Better, and less expensive in the long run, to use airsoft or paintball for "do-it-yourself" training...