Here's what you'll need

Typical bottle sizes are 5, 10, 15, 20 lbs. I have a 20 lb bottle which lasts for a very long time even though I make a lot. You'll need a regulator, dual gauge with a valve preferably. Mine goes to 40 psi. I wish it went a bit higher, but 40 is fine. You'll need a hose and a ball lock connector. Other than tephlon tape and a wrench, that's it. You can google these things and you might find them cheaper. I don't recommend buying a used tank because you don't know what's been in it and they need to be inspected every 5 years (I think), or you can't get them filled. Used regulators are OK. You might find the tank and regulator locally cheaper. Check with gas companies, welding supply shops, and home brew supply shops. You may want more than one carbonator cap. I have 3. There are videos on YouTube which show how to charge your bottles. Make sure you remove the plastic ring from used PEP bottles or you wont get a good seal. This same setup can be used to charge home brew beer kegs. You can also carbonate home brew in 2 liter bottles the same way as soda. If you buy commercial soda in PEP bottles, you can recharge leftovers and they don't go flat.

http://www.northernbrewer.com/shop/brewing/kegging/tubing/tubing-kits/gas-connector-kit-bl-ball-lock-version.html
http://www.northernbrewer.com/shop/co2-cylinder-10-empty.html
http://www.northernbrewer.com/shop/dual-gauge-regulator-w-1-4mfl-backcheck-outlet.html
http://www.northernbrewer.com/shop/the-carbonator.html