I don't know of any shops in Summit that do it. Here
is a map of dive shops in the Denver area.
Pretty sure any of them will do a refill for you. The Underwater Phantaseas place is a fairly quick on/off of I70 heading up towards Summit. Maybe 10 minutes off of the highway, probably not even that far. BA may know of some spots in Summit you can call to see if they'll do it. The other place that could lead you in the right direction is Wilderness Sports
in Summit. They sell avalanche gear. I doubt they do refills, but they will probably know where you can get it done. All of the mountaineering shops in Denver quit doing refills because of liability. The fact that they are not certified like dive shops are scared them off. There is also a dive shop close to my house. I am planning on checking with them about doing refills here soon. They would be more out of the way from the airport though than phantaseas is for the direction you are traveling.
Oh and airsoft guys are another one that can usually refill airbag canisters. Evidently Sports Authority is an Airsoft supplier. So maybe they can do it, and they do have a shop in Silverthorne. You might give them a call too.