I have a 5lb tank, and nearest refill station is 70miles away. Even at that, you have to go when they are refilling. The only places closer than thst only do 20 and 50# exchange.
I do this on some of my kegs, i use 1/2 the sugar as a bottling batch, leave it alone for 3 weeks at least, place in keggerator for 3 days, perge extra co2 presser off keg, hookup co2 to keg and pull a pint so you can't get beer back feed into your co2 hoses, just incase the keg is a bit to full.
I just filled my 5# tank today for $19.60 @ a locale fire extinguisher company. If you haven't looked into them to refill your tank do so because I had the same problem with small tanks at regular gas suppliers.