Any head shop or porno shop should carry a cracker...
Any place where they carry EZ whips, and the whipped cream dispensers mentioned above are huge motherfuckers, personally only good for doing multiple cartridges, but I just use one of the little handheld metal ones, a small cylinder about the size of your plam, easy to carry and load a few up in the car etc etc....
Those big fuckers are for home use, and personlly I am not a huge whippet fan (it's a supplemental drug in my opinion, just decent by itself, but when rolling or tripping it adds worlds to the effect) and the little metal cylinder will only cost you 7 bucks and do you just fine.
They also sell a cracker that is actually a bike air pump or something, it comes with a little cartirdge of CO2, toss that, and use it as a cracker. Personally these suck, but they are usually only 5 bucks and manage to work just fine....
Anyways, like I said, any porn store or head shop, ask for EZWhips and a cracker/charger.
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Lying on the stained wretched sheets with a bleeding virgin, we could plan a murder or start a religion...