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'.

2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car
$325.00
Time Remaining: 6d 16h 16m
Buy It Now for only: $325.00
Bachmann on30 Colorado Southern Flat Caritem 27323
Bachmann on30 Colorado Southern Flat Caritem 27323
$37.34
Time Remaining: 13d 10h 15m
Buy It Now for only: $37.34
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: 27d 14h 24m
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 on30 Colorado Southern Flat Caritem 27323
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car