Here are the locations where you can find the trading cards.

Kwik-E-Mart: In the Captains pipe in the display.
Simpson House: In Magies Room under the crib across from the monkeys paw; Outside the window in the tree at Bart's treehouse.
Flanders House: Behind a book in the bookshelf.
Retirement Home: In the upper right hand corner there's a little book shelf, and the card is between some books.
Krusty's Studios: In the control room behind the clipboard on the wall
Krusty's Studios: Behind the newsletter above the fountain.
Krusty's office: In the TV.
Library: In the garbage can.
School: In the principles office in the tape player in the bookshelf, not the one on his desk.
Church: Behind podium.
Power Plant: In Nuclear Core by light switch on the left.

"Yes, yes, you have just purchased the exclusive right to operate my Frink-a-Sonic Matter Transporter, mmoohey....! In order to use this device it should be noted that the Matter Transporter works as a network of ingenius devices set up in various, key locations throughout the town of Springfield. Now , to activate the Transporter, mmmbleek, you must open your Map , hold down the SHIFT + V + S keys (simultaneously, ahem!) and double-click on the desired destination star. Then in a whoosh of scientific miracle making, with the calculations and the adding and dividing, carry the one, mmoohey, you will be instantly teleported to that location! However , I cannot emphasize enough the importance of treating this privilege with the utmost caution -so please keep all hands and arms inside the unit during teleportation and do not leave any refuse in the main compartment."