If you need new S-6 tires, I recently shopped and settled on a "kit" from thebikesmiths.com - 2 Kenda tires, tubes and rim strips for $52 + $7 shipping. They also sell on eBay and/or Amazon but their website had the lowest total cost.

As far as I could find, Kenda is the last company making S-6 sized tires.