The best thing to do is make your own studded tires from the old tires you have. You can buy roofing nails for very cheap, and put lots of studs on the tire (most commercially available studded tires don't have that many studs).
Push the nails through from the inside of the tire, wherever you want them. It's a good idea to put some on the rolling surface, and some on the sides for cornering. Once you have placed all your nails, put a layer (or two) of duct tape over the heads of the nails to protect the tube. You'll probably need to trim the nails down to the desired length with metal clippers. That's all there is to it!
EDIT: I have had luck with nails in the past, but on looking around at studded tire designs online, I realize that screws may in fact be a better option, as they will better resist backing out, and would be easier to replace. Scroll down on this page for DIY instructions.