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: Skeeter Pee
Secondary: "Not a Chardonnay", Malbec
On Tap: Pale Ale, Brown, Marzen
Next batch(es): Cider, dirty blonde