Check out Great Lakes Casters - now known as Caster Depot...
http://www.casterdepot.com
I've always had good luck finding something in their clearance area for real, real, real cheap.
Also keep in mind that the larger the wheel is, the easier it will be to roll the cart.
I made my first cart with 2" wheels and it was too hard to roll around.
I changed over to 4" wheels & it made a world of difference.
I bought some 12" pneumatic wheels from Harbor Freight - but - never got around to using them.
My plan was to use the 12" wheels and roll the whole cart out into the driveway so I could unload right from the back of the van onto the cart.
I ended up selling the van so,,,,,