Also, if there is any new construction in your area, new house, etc..... When they stake out the house for position of the house and elevation on the ground floor, the surveyors will mark on a tree or other immoveable object a benchmark elevation above sea level. If you catch the job while the block layers or workers are on the site, they may be able to tell you where that benchmark is. Believe me, every new house going up will have this marked somewhere.
As far as airports, the official elevation of the airport is really just an average since they occupy so much land. If you call a NOAA weather location or an FAA Flight Service Station (FSS) located in the Blue pages. They can tell you the elevation of any airport in the area. They are usually there 24/7.
Thinking more on this, you can probably call an engineering firm or surveying company and they can tell you a location of a benchmark they use.
"Indecision may or may not be my problem"----J. Buffett