I've asked places that fill up the paintball bottles, before. They cannot do anything over a 24oz bottle without a pump. Some places have a pump setup, but many just have large CO2 tanks that they use.
IF you're not going to get a pump setup, I would just set up to fill the paintball size tanks. IMO, you'll upset more people with partial fill attempts of 5# tanks. Even charging less won't help when they run out faster. IMO, you'll probably have people either not get tanks filled, or not come back after one fill. Also, unless you have it plastered in BIG letters that a 5# tank is only getting about 4# of CO2, you're in for unhappy people. Even then, you have to remind each and every customer before, and after, you fill it up that it's a partial fill. IMO, really not worth the pain. IF you do get the pump setup, and offer to fill tanks for what many of the fire extinguisher places charge, you'll probably get a good amount of customers. Especially if you have better hours (later at night, open weekends, etc.)
See if you can lease/rent the compressor setup from the gas supplier. It might be an option that won't break your budget.
