Polybelt.com for a 40 inch long by 2 inch wide belt
vxb.com for bearings
sciplus.com for a motor (I used #40588, a gear motor intended to move automobile seats)
Files for the 3d printed parts are available at:
Start by making a "roller," 2 inches wide with a 2 inch diameter. I used a 1/4 inch diameter wood rod for the axle.
If you're smart (not like me), you'll put the roller guides on the rollers at this step--to keep the belt from slipping off.