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: 13h 19m
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: 3d 15h 9m
Buy It Now for only: $37.00
Durango Press Hon3 Westside Flat Car Handcar Plow Flanger DP 22 42 53 Kits
Durango Press Hon3 Westside Flat Car Handcar Plow Flanger DP 22 42 53 Kits
$40.00
Time Remaining: 8d 18h
Buy It Now for only: $40.00
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 433
ON30 AMS Flat Car Nevada County Narrow Guage NCNG 433
$45.00
Time Remaining: 19d 2h 41m
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: 19d 2h 43m
Buy It Now for only: $45.00
Triangle Scale Models Sn3 6200 series Flat Car Kit FREE SHIPPING
Triangle Scale Models Sn3 6200 series Flat Car Kit FREE SHIPPING
$25.00
Time Remaining: 28d 13h 38m
Buy It Now for only: $25.00

Other On30 Scale train items recently purchased through this site:

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
Durango Press Hon3 Westside Flat Car Handcar Plow Flanger DP 22 42 53 Kits
Triangle Scale Models Sn3 6200 series Flat Car Kit FREE SHIPPING