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
$350.00
Time Remaining: 1d 12h 7m
Buy It Now for only: $350.00
HOn3 WEST SIDE LUMBER CO 24FT BASIC FLAT CAR
HOn3 WEST SIDE LUMBER CO 24FT BASIC FLAT CAR
$25.00
Time Remaining: 15d 16h 59m
Buy It Now for only: $25.00
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 9h 2m
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
HOn3 WEST SIDE LUMBER CO 24FT BASIC FLAT CAR
2 car set 1203 Scale Narrow Gauge Fall River Gondola flat car