Welding shops, fire extinguisher refillers, and paintball stores all fill Co2 tanks usually. For me, I called every store in the area that I could think of and National Welders ended up being the best deal. $16 for a 20 lb exchange. Be sure to shop around because the price varies widely. Some places wanted more than $50 for the same 20 lbs.
As far as the 'grades', I think the only difference is what container it comes in, straight steel versus glass lined, for example. Also, I think most places only stock one type and go ahead and use 'food grade' so they can sell to more people.
EDIT: I was pretty wrong about the grades part. I found this on another forum:
FYI, i work in this industry. There are two main types of CO2, "Beverage Grade" and then everything else. The difference between the two is that beverage grade CO2 has less TRS, and HC, gasses in it, such as H2S, CS2 ect... Basically stinky gasses that make your soda smell like farts. Anyway, almost all CO2 that is used is beverage grade CO2. The stuff that isn't beverage grade gets turned into Dry Ice. Most plants that have CO2 as a byproduct run it through a lot of filtering processes to make it nice and clean "beverage grade". Most Welding CO2 is Beverage Grade as places that make CO2 always try to make the best stuff they can. Only if something breaks or doesn't work correctly do they have non-beverage grade CO2.