Check out Skyway wheels. The bead-lok line looks promising.
http://www.skywaytuffwheels.com/products_002.htm
They make a wide range of wheels though. The ones that sprung to mind are intended for powered applications like robotics. Their tread is designed to produce traction on a wide range of materials, supporting weights in the neighborhood your talking of. They have a good range of sizes and hub options.
Besides that, they are (relatively) cheap! The folks there have been very good to FIRST teams in the past and seem to genuinely like dealing with small guys like us. Mention its for a robotics project and they will probably get into it. You may even be able to talk to their engineers about their products, and get their view.
-Andy A.