You can really use almost anything to be the reservoir however I have found that a DU-BRO fuel tank works really well. You can pick one up at your local hobby shop or here. I would suggest you get the 10oz or 12oz one, you can go bigger but there is really no need.
Your DU-BRO fuel tank is intended for use with a RC plane or car, so it comes with a lot of parts you do not need, however hang on to all of them because we are not going to have them go to waste. Unscrew the 1 screw in the top of it and shake everything out. Read more at OCModShop.