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
$45.00
Time Remaining: 4d 18h 18m
Buy It Now for only: $45.00
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: 4d 21h 19m
Buy It Now for only: $260.00
On30 Blue Mountain Models 28 Ft Flat Car Kit SRRL
On30 Blue Mountain Models 28 Ft Flat Car Kit SRRL
$25.00
Time Remaining: 6d 9h 54m
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: 22d 15h 27m
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
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car
On30 Blue Mountain Models 28 Ft Flat Car Kit SRRL