Let's start from the beginning.

A paintball CO2 tank (do not buy HPA-compressed air tanks) comes in several dimensions. From 3.5oz ( 100 gram) to 20oz (600 gram).
Adaptor and/or refill station. Any online paintball store can sell you those.

Regulator. I believe there will be new Trevor s soon. Cheap (for a regulator) small, light and specific for RW. (use the search function)

Valves. you need at least 3 valves. Main control valve (this does the work for the weaopnactivation) , pressure relieve valve (safety , to vent overpressure) in a LP system it could be you need 2 of'm, and a dump valve (in most cases this is a small ballvalve)

Best bet is to search on Ebay. Because new pneumatics are expensive.

Other solution is to get custom stuff.