You can get super-tubes for $5 each at Niagaracycle. www.niagaracycle.com
I don't use the slime or any sealant either. Just get some tires with some tread thickness or puncture belts, and the thick tubes ("thorn resistant" tubes). They help cushion the ride more too.
Originally Posted by GearNut
The expanding foam sold at home improvement stores cures to a brittle state. It will crush inside the tube when weight is applied to it.
Your best bet is to use the $15 a piece "for down hill racing" mountain bike inner tubes which are very thick walled and install quality tire liners as well. The cost is high, but the dependability is high too. Ya get what ya pay for.
Some folks like the Slime brand self healing inner tubes instead of the down hill tubes, but I am not a fan of the sticky goo slime product.