You could probably use a small air compressor to fill the bag. Im not sure if you'd need pure oxygen to fill the bag, however most HVAC parts wholesale stores carry oxygen tanks for use with Oxygen / Acetylene torch sets we use for copper pipe fitting. They sell the tanks with contents, and also when the tank is empty we return the empty tank and get new full tank so only having to pay for "refill" content (similar to gas grill propane tank exchanges). You would need a regulator for the tank also so that you can safely adjust the pressure output. The regulator could be bought at the HVAC wholesaler or from the internet, however make sure if you buy online that the regulator has the proper certifications. Some HVAC wholesalers are contractor only sales but you could call and ask first.
Oxygen is fairly dangerous, be sure to take all safety precautions when using.