Where do I get constant-speed bulldozers for lab?

Georgian Mark DiBois asks:

One weird thing... I still can't find a small toy bulldozer to pull a mass across the table... Where do you get those?

Simple answer:  In the science supply houses you're looking for what's called a "constant speed vehicle."  Fool around on google and you can see the device pictured above, available from Frey, Fisher, and others for $25-30 each.  I *think* these can move forward or backward at the flip of a switch.

Better answer:  PASCO offers two levels of constant speed items.  In a rare event, PASCO undercuts the competition with their "constant speed buggy", pictured here.  These are $8 each.  They only move one direction, but they work just as well as the more expensive bulldozers.

Coolest answer:  If you have a few bucks lying around for luxury purchases, try out the PASCO "variable speed motorized cart" pictured here.  It lists at $159.  But it has a knob for adjusting speed, and has traction enough to climb reasonably steep ramps.

My personal solution:  I have two of the $25 bulldozers and ten of the $8 buggies.  (I like to have some of each because they move at different speeds.)  I use these for laboratory investigations.  I also have one of the fancy-pants variable speed carts, which I use exclusively for demonstrations, or even research problems where necessary.

GCJ