ive seen some instructables that use soda bottles
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This instructable has an easy to make air tank : www.instructables.com/id/How-To-Make-An-Air-Cannon-In-5-Easy-Steps/
check out this Instructable https://www.instructables.com/id/25MM-Pneumatic-Sniper-Rifle/ . It shows how to build an air gun. Just take the air tank's design, should be step 2. Make sure you use pressure rated pvc.
ok erm try get a empty c02 cartridge for b guns then make a thread on the top for your gun then drill a hole in the side of the cartridge and fit a bike pump thing to it then pump it up and thread it onto your gun