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'.

On30 Bachmann Spectrum 26511 18 Flat Cars Painted  Unlettered 2 Box
On30 Bachmann Spectrum 26511 18 Flat Cars Painted Unlettered 2 Box
$60.00
Time Remaining: 14h 58m
HOn3 DRGW 6500 SERIES FLATCAR KIT BY PRECISION SCALE CO
HOn3 DRGW 6500 SERIES FLATCAR KIT BY PRECISION SCALE CO
$60.00
Time Remaining: 6d 3h 15m
Buy It Now for only: $60.00
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: 6d 8h 30m
Buy It Now for only: $260.00
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
$55.00
Time Remaining: 15d 6h 17m
Buy It Now for only: $55.00
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: 25d 8h 21m
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
HOn3 DRGW 6500 SERIES FLATCAR KIT BY PRECISION SCALE CO
On30 Bachmann Spectrum 26511 18 Flat Cars Painted Unlettered 2 Box