It depends on how realistic you want your walls to be. To duplicate a wood-framed building, you can recreate the full-scale elements exactly (studs, sheathing) with basswood or balsawood strips and sheets scaled to size, and built up just like a full-scale building (floor plates, vertical studs, top plates and outer sheathing. The inside can be left bare, like you would have in a typical residential garage, or covered with cardboard or sheet styrene "drywall." (of course, if you're going to cover the walls on the inside there's no point in building the detailed stud walls, as they won't be seen).
If the look you want is brick, there are scale brick-patterned styrene sheets available from Plastruct and/or Evergreen. Check out their websites for more info. "Brick" walls can also be made by scribing your "mortar" joints into thin foam sheets, then painting the wall to look like brick. Tedious and labor intensive, but pretty realistic if you do it right.
If you're near a Hobby Lobby, check out the display of hobby woods they have... it comes in a ton of different sizes, and you'll almost surely find what you need.