Although I chose to build a cart for myself, here are a couple of links
I found for street organ carts:

Custom Made Street Organ Carts:

  The Great Canadian Nickelodeon Company, Ltd.
  http://members.aol.com/tgcnc/customcart.htm

Carts which fit their organs:
  Dean Organ Builders
  http://www.deanorgans.co.uk/neworgans.htm

Cart which fits their organ:
  Ragtime Automated Music
  http://www.ragtimewest.com/we12000.htm

Todd Augsburger - Roller Organs
http://www.rollerorgans.com/