If you don't know which cards you're missing, there's always 3 cards per level, so 9 cards per world.

1-1
2nd room

3rd room right in front of the entrance

4th room after the second mushroom


1-2
2nd room, high up where there's 3 blue dude

3rd room right above the card to summon a vine

4th room next to the really big vine


1-3
2nd room to the left side of the exit
3rd room on the right side of the map
2nd room secret exit (also tells you about the secret exit on the right side of 1-3 first room)

2-1
1st room, on the left side
2nd room by following the yellow balls upper path
3rd room bottom right, a little to the left of the exit

2-2
3rd room when going down after the exit
4th room, when using the yellow ring to bypass the blizzard, hold right as to not get into the yellow ring that throws you down. Then go up and right to find the card
5th room

2-3
1st room, walk on the last ice path, and before it goes up go left to avoid getting lifted up. A path will open letting you collect the card.
2nd room, behind the glass
5th room, near the end of the up/down wind

3-1
1st room, on top of where there's a bunch of music blocks
3rd room, below the air blower
still in the 3rd room, use the music block to launch yourself in the air

3-2
2nd room, on the path below
still in the 2nd room, with the part with the drums, go up left
3rd room, take the upper path, you can start near the exit and go up left

3-3
2nd room, go right and then up the ramp (or go on the ceiling) the card will be there
3rd room, when reaching this place (after the first tube) use the cannon and wall bounce to go up where there's the npc, then walk on the ceiling to the left where there's the electric obstacle and then up some more to find the card
4th room, go up and right on this ramp (imagine the card is where I am)

4-1
6th room, on the right side
6th room, near the top left, a little to the left of the exit
7th room, on the top right on the second star, you can find a chick, which you can take to the gator gate right below the exit

4-2
3rd room, take the wheels until you reach a platform and enter the chest to get the card
4th room, right above the first gelatin carts
5th room, left to the second cannon

4-3
4th room near the bottom
5th room, near the entrance, you have to come from the bottom
6th room, when taking the final cannon

5-1
4th room, when at this spot, go up
6th room, take the right path when you enter the room
7th room, just before going down, hold right to not be blown away by the bubbles and grab the card

5-2
2nd room, on the bottom, a little bit before leaving the open area
4th room, above the water path
6th room, near the top left. You can go up as soon as you enter the room to catch the bubbles and save some time

5-3
1st room, go down where there's the first stone block and 3 hats
3rd room, in the middle
7th room, take the right water path at the end, the left one is the exit

6-1
2nd room, go up using the star
3rd room, use the star to go up left
5th room, after climbing the first scrap wall, go left where there's 4 ennemies floating

6-2
2nd room, go down right after the initial corridor
4th room, go right (below the exit)
5th room, fall into the first chasm but hold right to activate the bumper, than spin to get the star and keep going up so you can get the bumpers and activate the second star, then simply follow the path

6-3
in the middle of the second level where you have to collect ducks
in the rythm game, go up on the left side of the wall. There will be arrows that you can use to launch yourself up
after the arcade room, go left near the end

7-1
all three cards are behind the three small doors you can find in this room

7-2
3rd room, around the middle of the room, you start seeing vines like so. Stay on the bottom path and take the huge vine
6th room, at the very top, take the bottom path in the water
8th room, left to the exit where there's a huge column of baloon

7-3
2nd room, after the second star, don't follow the balls and instead stay near the bottom
3rd room, stay on the left side
5th room, to the left of the star used to finish the level