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'.

On2 SANDY RIVER CAR SHOPS KIT 101 WWF 30 FLAT CAR
On2 SANDY RIVER CAR SHOPS KIT 101 WWF 30 FLAT CAR
$30.00 (1 Bid)
Time Remaining: 7h 48m
CREATIVE CONVERSIONS A BACHMANN On30 FLAT CAR converted to a PULPWOOD CAR
CREATIVE CONVERSIONS A BACHMANN On30 FLAT CAR converted to a PULPWOOD CAR
$75.95
Time Remaining: 8h 18m
Buy It Now for only: $98.75
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: 1d 55m
Buy It Now for only: $37.00
NEW HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR FLAT CAR
NEW HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR FLAT CAR
$39.99
Time Remaining: 7d 10h 48m
Lot of TwoHOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50
Lot of TwoHOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50
$25.00
Time Remaining: 9d 21h 39m

Other On30 Scale train items recently purchased through this site:

On2 SANDY RIVER CAR SHOPS KIT 101 WWF 30 FLAT CAR
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
NEW HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR FLAT CAR
Lot of TwoHOn3 Durango Press Westside Lumber Flat Car Unassembled Kit DP 50
CREATIVE CONVERSIONS A BACHMANN On30 FLAT CAR converted to a PULPWOOD CAR