If there is a league around you, participating in it can get you a good supply of popular cards, three of them being Spiritomb (Arceus set), Expert Belt (Arceus set), and Azelf (Legends Awakened set).
In the past cycle, the league also had Uxie (Legends Awakened), Bebe's Search (Rising Rivals set), and Warp Point (Majestic Dawn).
Warp Point is a reprint from Gym Challenge and later Expedition and various other sets so you may have it in any old cards you have. Reprint rules allow you to use the old releases.
When it comes to getting Pokemon Collector and Pokemon Communication, 2 very good supporter and trainer cards, respectively, in the game right now, you'll probably be better off getting one of the theme decks that has at least 2 copies of one, if not both, in it than trying to trade for them. The Typhlosion deck from HGSS has one of those. I think the new Call of Legends theme decks also have them.