Tropical Beach was the Worlds Promo for 2011 and 2012, as you're aware. It could be obtained in a few ways:
~Being eligible to play the Swiss at Worlds on Saturday, either by grinding in or receiving an invite (would have just the basic Worlds 11/12 stamp, like all the Beach cards)
~Staffing at Worlds that year (would come with a Staff stamp)
~Making it to a certain point in the top cut on Sunday, top 32 or higher (would come with a Top _____ stamp as well). Obviously, the higher up you go, the more they increase in worth.
All Tropical Beaches in existence ultimately come from those sources (primary sources). Anyone who's offering to sell or trade theirs got it from somewhere else (secondary sources).
Oh, and as an aside, that is technically not a rulings question. This would have fitted better in TCG General, fyi.