Where can I get a CO2 tank refilled in manhattan?  The further
downtown the better.  I've heard welding supply stores are the place
to do this, but I figure other stores must have this capability as
well, right?  The C02 tank is a smaller one, for a single-keg
kegerator.    Thanks, Matt

Hi Matt,

You can get you CO2 tank refilled at Able Fire Extinguisher CO, located at 

"Able Fire Extinguisher CO
241 West 26th Street Floor 2, New York, NY 10001 
212) 685-8314"

Here is the driving map for your driving convience [its a big URL]:

http://yellowpages.superpages.com/supermaps/mapinit.jsp?LID=0002001337&SRC=google&lat=40750565&lng=-73996209&POI1name=Able+Fire+Extinguisher+CO&POI1lat=040750565&POI1lng=-073996209&streetaddress=241+West+26th+Street+Floor+2&city=New+York&state=NY&zip=10001&SRC=google&C=Government+Offices&T=New+York&S=NY&PP=N&CID=00000525880&MAPWIDTH=800&MAPHEIGHT=600&level=8#Map

Able Fire Extinguisher COs' bussiness profile:

Products & Services:
All Major Brands 

Additional Information:
Sales, Service, Recharging, WE DON'T OVERCHARGE, 241 W 26 Manhattan,
Rentals Available Immediately, Violations Removed, Fire & Safety
Equipment, A Complete

Hope this is what you are looking for.
cheers
Arun