Airgas.... but they were ridiculously expensive for a 5lb tank.. also they don't fill they swap, they have a bunch of locations though so they are convinient.
Nobby Bev in Plainville, CT does refills, not sure of price yet; but now that I have a 20lb aluminum tank that is pretty I'll be going there so I can keep my pretty tank :-)
Primary: Belma Blonde, Narzen, "2-buck-chuck" wine
On Tap: Workaholic SMaSH Blonde, Brown, Cider
Bottles: King Czar Stout, Wee Heavy Scotch Ale, Christmas Lager
Aging: RIP RIP Stout (Rest In Peace Russian Imperial Stout)
Next batch(es): "two-buck-chuck" wine, Oktoberfest, Belma SMaSH