When Lass is played: Back then, there were no distinctions on Trainer cards. When Lass is played, both players will return ALL Items, Supporters, AND Stadiums to the deck.
When Muk is around: As per the Compendium LV.X, anything that affects Pokemon Powers will affect both Poke-Powers and Poke-Bodies, and the reverse is also true: anything that affects both Poke-Powers and Poke-Bodies will also affect Pokemon Powers. If Muk is in play, ALL Pokemon Powers, Poke-Powers, and Poke-Bodies will be negated, except for those named "Toxic Gas". Any Abilities, however, will be unaffected; they are distinct and separate.
When Item Finder is played: Same reasoning behind Lass. There was no distinction on Trainer cards. Thus, you may use Item Finder to return an Item, Supporter, or Stadium from your discard pile to your hand.