These companies that sell the "Pet Oxygen Masks" make them out to be a revolutionary, rare, product. However. Any vet surgical supply business would have them. They're for resuscitation and anesthesia, they've probably been around for decades. They even make them for animals the size of mice, or as large as a horse or cow. But they're expensive as all hell. If you're just looking to give oxygen, you can probably make a mask easier than buy one. Just leave it somewhere it won't come apart, and mark on it what it's to be used for.
Harvard Bio Science, Harvard Apparatus; sells them. (for small animals, I should mention. Look at vet supplies for larger animals, if you need a big one.)
http://www.harvardapparatus.com/webapp/wcs...egories_N_37326
http://www.harvardapparatus.com/webapp/wcs...51_10001_-1_HAI
You can also order them from any Valley Vet Supply; or Tractor Supply Inc., retail store.