Jasper that is a great find I decided to bite the bullet and move up to a 20lb from a 5lb!
Here is the list of what I have found so far:
Liquor Mart - Boulder: 5lb exchanges only
Airgas - Boulder: 5lb, 10lb & 20lb exchanges only
Wagner Welding Supply - Longmont: Walk in exchanges or drop off refills. They have 20lb on hand for exchanges and a 20lb exchange/refill is $12.95.
For me it looks like Victory Hydro is the place since it's here in Louisville.