We've searched the internet and found the largest collection of On30 Scale trains for sale is on eBay. Below are the listings for 'tank car'.

SPECTRUM 27199 ON30 Tank Car Black Painted Unlettered Bachmann
SPECTRUM 27199 ON30 Tank Car Black Painted Unlettered Bachmann
$49.95
Time Remaining: 5d 4h 23m
Buy It Now for only: $49.95
On3 On30 WISEMAN MODEL SERVICES WEST SIDE LUMBER CO OIL TANK CAR 5 KIT
On3 On30 WISEMAN MODEL SERVICES WEST SIDE LUMBER CO OIL TANK CAR 5 KIT
$89.95
Time Remaining: 5d 13h 3m
Buy It Now for only: $89.95
Silver City  Wiseman On30 4 Wheel Flat and Tank Car kits
Silver City Wiseman On30 4 Wheel Flat and Tank Car kits
$35.00
Time Remaining: 17d 7h 48m
Buy It Now for only: $35.00

Other On30 Scale train items recently purchased through this site:

SPECTRUM 27199 ON30 Tank Car Black Painted Unlettered Bachmann
Silver City Wiseman On30 4 Wheel Flat and Tank Car kits
On3 On30 WISEMAN MODEL SERVICES WEST SIDE LUMBER CO OIL TANK CAR 5 KIT