Plenty of real-estate investors are poised to buy property in whichever city Amazon.com Inc. picks for its second headquarters. Some aren’t waiting.

Speculators are raising funds to invest in real estate near the winning site—wherever that may be—or are gathering cash commitments so they can pounce immediately after the winner is announced. Others are buying up shares of a real-estate firm that owns much of the property in a north Virginia city that many consider a leading contender.