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'.

Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
$49.99
Time Remaining: 2d 10h 46m
Buy It Now for only: $49.99
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: 5d 12h 35m
Buy It Now for only: $37.00
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP
$49.99
Time Remaining: 16d 6h 4m
Buy It Now for only: $49.99
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 433
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 433
$45.00
Time Remaining: 21d 8m
Buy It Now for only: $45.00
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 401
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 401
$45.00
Time Remaining: 21d 9m
Buy It Now for only: $45.00
bachmann hawthorne village BUDWEISER flat car On3O scale
bachmann hawthorne village BUDWEISER flat car On3O scale
$28.75
Time Remaining: 29d 16h 24m
Buy It Now for only: $28.75

Other On30 Scale train items recently purchased through this site:

bachmann hawthorne village BUDWEISER flat car On3O scale
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 433
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 401
Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP