Many farm supply stores sell individual cross links and chain by the foot. If you're comfortable cutting, splicing, and measuring chain links you should have no problems extending your chains to fit a bigger tire. They make a special clamping/spreading tool just for tire chain work that makes adding cross links a breeze. Do a search on Amazon for "tire chain tool" and you'll see several to choose from. I have the cheaper one and it just does the job for pickup or passenger car chains. Never played with tractor chains, but I suspect you'll need the bigger tool if your chain links are really thick.