So I thought of an idea which incorporates split piles and Events.
So you have a normal pile of 10 cards, but once they are bought out, instead of having nothing there, a new event is available. It still counts towards 3-pile ending. If Ambassador etc causes those cards to be returned to the Supply, the event become unavailable until the pile is empty again.
For example:
Flea Market $2 Action
+1 Card
+1 Action
+$1
----------------------------------------------------------
When you discard this from play, take a Debt token, then you may pay off Debt.
Vendor $5 Event
Gain two cards each costing up to $2 + $1 per Flea Market you have in play. If either of them is a Victory card, trash all Flea Markets you have in play.