For an approximation, Google Earth.
Draw a "Path" (Add -> Path) from somewhere at sea level to your intended destination and hit Edit -> Show Elevation Profile.
Tools -> Options to change it to feet if you desire that.
In the air, part of the fun I remember during PPL navigation exercises was a diversion to an unplanned airfield. You should get a feeling for circuit heights and standard circuits so that even navigating to a unplanned airfield you can judge the circuit height and therefore approximate the airfield height even without any other information and then get ready for your ALA inspection.