I've read Woodcraft is a good supplier of 1/4 and 1/8 inch SC lumber. I also read these guys offer great pricing for DIY walkins :
http://www.woodprojections.com/walk-in%20humidors.htm. I found this supplier of 1/4 SC lumber at $5.99/sq ft:
http://www.woodworkerssource.com/hum...r_Spanish.html
Per Scott's post I would steer clear of anything ply. Go with 1/8 inch thick lumber to cut costs. FWIW, make sure it's kiln dried or you may have sap issues.