We've searched the internet and found the largest collection of On30 Scale trains for sale is on eBay. Below are the listings for 'flat car'.

Smoky Bottom Lumber Co Short Flat Cars With Trucks Unbuilt
Smoky Bottom Lumber Co Short Flat Cars With Trucks Unbuilt
$40.15
Time Remaining: 1d 21h 15m
Buy It Now for only: $60.23
Bachmann Spectrum ON30 Set Of 2 Flat Cars
Bachmann Spectrum ON30 Set Of 2 Flat Cars
$42.96 (2 Bids)
Time Remaining: 1d 21h 15m
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
$37.00
Time Remaining: 28d 14h 9m
Buy It Now for only: $37.00

Other On30 Scale train items recently purchased through this site:

On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
Bachmann Spectrum ON30 Set Of 2 Flat Cars
Smoky Bottom Lumber Co Short Flat Cars With Trucks Unbuilt