Both Bachmann and AMS make 1:20.3 scale rolling stock that matches their Shay. If you're into kit building, you can also get suitable rolling stock from Northeast Narrow Gauge in Wiscasset, Maine.
They take some time to make but Hartford Products makes several kits for logging cars in 1:20. Check out their site at
http://www.hartfordpr.com/
If you are running tight radius curves, you may want to call and see what size curves each kit can handle.
I've got a train of these for my Shay.
http://1stclass.mylargescale.com/JoeJohnson/Log%20Car/WholeCar.jpg
Like kstrong said, Accucraft (AMS) is coming out with some log cars very soon, and the pricing actually isn't too bad considering trucks and couplers are included. Best prices I have found so far are at http://www.rctrains.com/Rolling_stock%20AMS.htm. I am not associated with them in any way - I just googled it.
I may have to save up and buy a few of them short ones...
Scott
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