I'm no expert but a 20# tank would meet your needs of waiting longer between refills but a 10# tank might be a better alternative.
as for refills look for welding supply places as well as anyplace that refills paintball tanks. I'm looking into a good set up for my tank but have a few months before I have the extra cash to drop on one.
I'm sure if my info is wrong someone will correct me.
pets warehouse has CO2 tubing as well as the petco in carl place