The best market now of days is Flat Track Swap & Shop on Facebook
https://www.facebook.com/groups/357166127665643/
Vintage Flat Track Forum which always has a good selection of flat track stuff for sale
http://www.vft.org/vftforsale2.html
The other alternative forum for flat track classifieds is
https://www.facebook.com/flattrackclassifieds/?fref=ts
Cheney Racing Products
http://www.flattrackaccessories.com/
Flat Track Racing Frames
Curtis Racing Frames – http://curtisracingframes.com/
Panther Racing Frames- https://www.facebook.com/Panther-Frames-1267151126691657/