I was curious about this, too.
Using a simple google search with key words "Airnav" and "Bottled oxygen: HIGH", I was able to generate a list of FBOs that purport to sell oxygen. I also added state names to narrow the lists to places I might want to visit.
I have no idea how up to date the information might be.
Sorry - Asked the question wrong. I know I can look at each airport under services. Question is - Is there a way without looking at each individual airport.