Nitrous Oxide

Well in the states I guess you just go into your local head shop. If you can't you can adapt a soda syphon like http://tinyurl.com/jpfsh all you need to do is put a little bit of plastic from a carrier bag on the hole where the sphere screws into the top bit (to stop the gas going into the sphere) and screw it up. You then just need a bit of string or something to hold down the lever.

So to do a whippet just put a balloon on the nozzle put a whippet into the charger thing thats comes with it (see picture) and screw it up.

charger thing = BULB SCREW in the picture on ebay btw

how about this? http://www.erowid.org/chemicals/nitrous/