Individual pieces may have been sold by some hobby shops, but I’ve only ever seen it offered by stores in the factory box quantity (usually 6 to a box). Good luck with your search...
Bill in FtL
S-Trax comes in three radius, 20, 25, and 30" ( S-Helper used to call them 19, 24, and 29"). You can verify which you have by counting the number of ties. 33, 42, and 51 respectively. Portlines has used single 29" radius curves for sale for $4. Choose "Track Options" >> "Find Inventory" from the menu bar then look for "SHS-294-CRV-29" about 3/4 down the page.
They also have a few 15" straights "SHS-257-STRT-15" for $6 and some 10" straights "SHS-258-STRT-10" for $4.
SHS track is now made by MTH. So if you decide to add a lot more you can buy the track and accessory pieces from MTH.
Thanks everyone. I just ordered a set from Western Depot.