Go get some dry ice. It's readily available and cheap. Siphon 2 liters of beer into an empty 2-liter pop bottle, filling it almost completely. Weigh out 7 grams of the dry ice. Get your cap ready, drop the dry ice into the bottle, slap the cap on quickly, shake the heck out of the 2-liter bottle until the dry ice has completely sublimed. You'll feel the pressure go up in the bottle almost immediately. (when you drop the dry ice in, it'll bubble vigorously, thus clearing the headspace of any Oxygen.)
INSTANT carbonation!
There's only one drawback to this method. I had a 2-liter bottle explode on me once. It cut my hand pretty deeply when the bottle fractured.
If you try this, make ABSOLUTELY CERTAIN that you can put the cap on quickly enough to be able to shake the bottle so that the dry ice doesn't sink to the bottom and freeze there. If that happens, it will make the plastic VERY brittle from the extremely cold temperature, and it could rupture. Not a pleasant thing, I assure you!
Wear gloves and eye protection! Also, I suggest ONLY using Pepsi-product bottles. Apparently, they're a bit stronger than other manufacturers'.