1.1L tanks or 68ci (Cubic inches) can be bought from any good paintball supplier.
http://www.paintballwarehouse.co.uk/ theres a good place to buy one
Alternativly you can get a second hand one from anyone on the forums. Get the Fiber Warped tanks (Kevlar), they need to be tested every 5 years from born date and have a lifespan of 20 years. Get a bottle cover for it too, it should help stop the bottle from sliding off your shoulder whilst playing.
The new ETEKs from planet look really good if you haven't already bought your marker.
Also for the same price other guns are available such as the Proto PMR which shoots really well too
Happy ballin'