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: IPA, Coffee Porter, Pale Ale
Secondary: pinot noir
On Tap: apfelwein, and more apfelwein
Next batch(es): Pilsen SMaSH
"Oh no, not again" - A Bowl of Petunias