How does Swindler interact with Spoils (or other cards not in supply)?
The cost of Spoils is 0. When you 'gain(s)' a card, unless it specifically names a card not in the supply, you can only choose for cards from the supply.

Copper, Curse, and Ruins (when playing with them) are in the supply and cost 0. Making all valid choices for cards that can be gained when Swindler trashes Spoils. 

http://wiki.dominionstrategy.com/index.php/Cost


  Every card in Dominion has a cost indicated in its lower left corner.
  
  A few costs are listed with asterisks: 0* is used on
  several cards that can be gained from outside the Supply; the asterisk
  serves to remind players that they cannot actually be bought for
  0