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'.

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: 4d 12h 11m
Buy It Now for only: $40.00
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car
$260.00
Time Remaining: 24d 9h 5m
Buy It Now for only: $260.00
Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
$49.99
Time Remaining: 28d 7h 30m
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: 31d 9h 19m
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
Durango Press Hon3 Westside Flat Car Handcar Plow Flanger DP 22 42 53 Kits
Confederate Cannons Flat Car Hawthorne Village Civil War Confederate Express
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car