I got mine from a tank filling station originally working for the fire houses. They have the best deals and it is also crazy cheap to refill. As for the regulator, CO2 Art has the best performance/price in my opinion.
Airtech in North Dallas has 10g tank filled for $80 I believe. My regulator was $30 on Amazon. Since it's already filled, that probably counts. I've read welding supply places can refill, but didn't have any luck around here. Fire repair and supply facility near my work has the best refill price.
For an inexpensive regulator I us the up aqua ones (less then $70 on Amazon) have two of them, did have to send back and reorder my second, but once set it holds count. Second one I had ordered was defective, first hand third are perfect.
Going rate for a 5/10lb canister is 70-90 filled in NJ. I got one tank from a brewing company, one from a fire safety place... both cost about the same filled... make sure if you are buying a tank it has a recent manufacture date
Depends on your tank size which drives BBS rate and ppmCO2. Most go to 30ppm, but some go higher. My 54g tank at 30ppm needs about 3bbs, goes through 1.5lbs in about 3 weeks with 9 hours per day of gasing... Give or take.