When brainstorming keywords, start with specific geographic keywords and move to more general ones, as needed. For example, start with "Boulder" then move to "Boulder County", "Front Range", or "Colorado" if the previous term doesn't return enough results.
Add the phrases "aerial photo*" or "satellite image*" to your geographic keyword(s).
Search university library websites in the state your are interested in finding photos for to see if they have an aerial photo collection for that state.
Evaluate aerial photography sites closely to determine if photos are only available for purchase or if they are freely available.