The front wheel isn't original, looks like it's from a small kid's scooter or something.
Good source for a proper
looking wheel is the Harbor Freight 13" wheel SKU #37767 that's currently $11.99 with coupon #39378223
Caveat is, you don't want to use it as-is, because the tire and bearings are junk.
However, you can replace the cheap wheel barrow bearings with some flanged sealed units for little outlay:
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=400310122844
It looks like your rear tire is a 13 x 5.00-6
Carlisle turf tire (or copy), so you'd want something along those lines to replace the super-cheap tire that comes with the wheel assembly. The tube in there is decent, so you can keep that.
Keep in mind, you'll want to make an internal tube spacer for the bearings, and a couple wheel spacers to keep the wheel centered on the axle.