i posted something time ago about one used for divers (and see mike allen post too)

http://www.kayakforum.com/cgi-bin/Building/index.cgi/noframes/read/155656

Inflating this its orally or with the bc hose but as it turns out after some correspondence if you do the work you can adapt a canister for inflation (obviously if you work at it you can adapt anything to anything). The standard with a canister is to automatically inflate just enough to geth some rigidity and the final inflation would be topping off by mouth, since its impractical to have a big canister on you. Canister you could use are stuff for pellet guns or, if you want, the bigger ones for paintball guns, they are even the source for valves and reliefs. Keep the pouch attached to the PFD and it should be fine.

Truth is that you could use any small inflated device from big stores, and that it is actually possible to craft your own, that of course if you dont mind having a tad more weight and bulk since common material is different. If i had to make my own id be looking to include it in a sprayskirt.

Not long ago i found a few of the real stuff in Italy, Air Force surplus, they were complete with all the rigamarole of safety devices and addons, sort of cool to have one but the size of the packet and price that reflected the "rarity" made not worth to get one. But still they are a source.

Just to know what i was referring to, the site i mentioned for the sponsoon is http://www.canoekayaksafety.com if you feel like reading the whole thing... i still havent.