ESPN
X Games Skateboarding
Hard mode
Get all 36 licenses in arcade mode under normal mode, then press Circle at the stage selection screen.
Big head mode
Get 30 licenses in arcade mode with any character, then press Square + X at the character selection screen.
Bob Burnquist's board in L.A.
Get to the mansion. The board is on top of the car.
Rick's board in Museum
In the third room, go up the slope and grind the dinosaur to get the board.
Rick's board in Boat
Go to the first floor in the back of the boat on the grind rail.
Bonus boards
Unlock hard mode, then play a level until you get all of the licenses for that level. For every level you do this, you will unlock an extra board.