I just went to my local Home Depot, and they have a good selection of "shopping cart wheel" style casters in various sizes. They range anywhere from about 4cm in height to massive wheelbarrow size ones. They have 4 pre-drilled holes in the corners so you can attach it to something. It didn't take too long to find the correct bolt size and I was soon good to go. It works well, although it did need a good spray of WD-40 onto the ball-bearings to allow it to rotate smoother.This didn't cost me more than $2 CAD.
Although you can't find the products on their website, when I went there was tons of them sitting in different bins.
Note: I live in Toronto, Canada. I also found identical casters at a local Canadian Tire. Lowe's might have them too, but I can't confirm that.