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'.

SPECTRUM 27399 ON30 Flat Car Painted  Unlettered
SPECTRUM 27399 ON30 Flat Car Painted Unlettered
$39.95
Time Remaining: 4d 11h 21m
Buy It Now for only: $39.95
SPECTRUM 25398 ON30 Porter Steam Locomotive Painted Unlettered
SPECTRUM 25398 ON30 Porter Steam Locomotive Painted Unlettered
$99.95
Time Remaining: 5d 7h 1m
Buy It Now for only: $99.95
bachmann hawthorne village BUDWEISER flat car On3O scale
bachmann hawthorne village BUDWEISER flat car On3O scale
$28.75
Time Remaining: 12d 13h 32m
Buy It Now for only: $28.75
HOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50 NIB
HOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50 NIB
$25.00
Time Remaining: 14d 7h 26m
Buy It Now for only: $25.00
ON3 ON30 SILVER CITY MODELS DRGW 1872 14 4 FLAT CAR W LOAD BUILT
ON3 ON30 SILVER CITY MODELS DRGW 1872 14 4 FLAT CAR W LOAD BUILT
$55.00
Time Remaining: 15d 17h 11m
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: 18d 9h 43m
Buy It Now for only: $37.00
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP
$49.99
Time Remaining: 29d 3h 12m
Buy It Now for only: $49.99

Other On30 Scale train items recently purchased through this site:

On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
bachmann hawthorne village BUDWEISER flat car On3O scale
ON3 ON30 SILVER CITY MODELS DRGW 1872 14 4 FLAT CAR W LOAD BUILT
SPECTRUM 27399 ON30 Flat Car Painted Unlettered
HOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50 NIB
SPECTRUM 25398 ON30 Porter Steam Locomotive Painted Unlettered
Logging Crane small Log Loading crane for a loading dock or flatcar HO BUILT UP