$4.47 each for 2" rubber-roller omniwheels for robot drive applications: http://store.kornylak.com/searchresu...show=10&page=2
Be sure to get the CAT-TRAK style (comes in several colors/configs) with the rubber rollers for drive traction. They have cheaper wheels with slick plastic rollers which would not get any traction at all.
Several other types & sizes of wheels there, well worth a look. Plastic sprockets for #25 chain on page 1 go for just $1.11. I also like their 6" stamped steel mecanum wheel, but a tad pricey at $126.00.