Nico Nico Nii!~
Typically, with most cards, there's 2 variations of the same collector number within a set, but some cards get alternate prints in theme decks (holofoil rares get a non-holofoil print, while non-holo rares get a "refractor" holofoil print), and in checklane blister packs (some have a "refractor" foil, while others have the traditional foil, while still others have the new standard foil from the Black & White era sets).
That's typically called a reverse holofoil. Some secret and ultra rare cards share characteristics with a reverse holofoil, though.
There's not really a good place that lists all possible alternate prints in one place for any given set. The closest would be the setlists over on Bulbapedia (http://bulbapedia.bulbagarden.net), but those don't typically list all the alternate prints.
As for places that allow trading, I know there's a large community of YouTube traders, some other Pokemon websites allow trading, and wherever a league is nearest to you would probably be a good place to check for people who want to trade.