Scott, I've found that the stock tires, sanded true, work just fine. Another option if you want/need more grip is from Slot.It, they make a few compounds of tire for Ninco tracks. hHopefully the link below takes you to the table on the Slot.It site that I'm referring to.
http://www.slot.it/immagini/KitPart/tir ... omme-1.jpgI've not tried them b/c for my taste the stock tires are fine on most cars....most :D I do have a few cars I'd like to test with a different compound.
I have had luck with Paul Gauge tires on Ninco track.
Let us know what you go with and how it works out.