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

DURANGO PRESS HON3 WESTSIDE LUMBER CABOOSE 6
DURANGO PRESS HON3 WESTSIDE LUMBER CABOOSE 6
$35.99
Time Remaining: 6d 5h 28m
HOn3 10 CS CABOOSE 4 WHELL BRASS SLIM PRINCESS BY BALBOA SCALE MODELS
HOn3 10 CS CABOOSE 4 WHELL BRASS SLIM PRINCESS BY BALBOA SCALE MODELS
$90.00
Time Remaining: 6d 11h 41m
Buy It Now for only: $90.00
MOUNT BLUE MODEL CO SRRL RR CABOOSE 556 KIT 9
MOUNT BLUE MODEL CO SRRL RR CABOOSE 556 KIT 9
$79.95
Time Remaining: 11d 8h 48m
Buy It Now for only: $79.95

Other On30 Scale train items recently purchased through this site:

DURANGO PRESS HON3 WESTSIDE LUMBER CABOOSE 6
HOn3 10 CS CABOOSE 4 WHELL BRASS SLIM PRINCESS BY BALBOA SCALE MODELS
MOUNT BLUE MODEL CO SRRL RR CABOOSE 556 KIT 9