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

HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
$59.99
Time Remaining: 2d 9h 11m
Buy It Now for only: $59.99
Hawthorne Village Silver Moon Express Leader of the Pack Box Car Track HO Scale
Hawthorne Village Silver Moon Express Leader of the Pack Box Car Track HO Scale
$37.49
Time Remaining: 12d 5h 35m
Buy It Now for only: $37.49
Hawthorne Village Silver Moon Express Wolf Walk Box Car Railroad Track HO Scale
Hawthorne Village Silver Moon Express Wolf Walk Box Car Railroad Track HO Scale
$37.49
Time Remaining: 12d 5h 41m
Buy It Now for only: $37.49
Hawthorne Village Silver Moon Express Wolf Ridge Box Car Railroad Track HO Scale
Hawthorne Village Silver Moon Express Wolf Ridge Box Car Railroad Track HO Scale
$37.49
Time Remaining: 12d 5h 46m
Buy It Now for only: $37.49
Hawthorne Village Silver Moon Express Pack of Friends Box Car Track HO Scale
Hawthorne Village Silver Moon Express Pack of Friends Box Car Track HO Scale
$37.49
Time Remaining: 12d 5h 54m
Buy It Now for only: $37.49
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
$49.99
Time Remaining: 16d 7h 34m
Buy It Now for only: $49.99
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
$49.99
Time Remaining: 16d 7h 35m
Buy It Now for only: $49.99
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
$64.18
Time Remaining: 18d 18h 15m
Buy It Now for only: $64.18
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
$64.25
Time Remaining: 18d 18h 38m
Buy It Now for only: $64.25
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
$36.00
Time Remaining: 20d 5h 15m
Buy It Now for only: $36.00
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: 22d 3h 12m
Buy It Now for only: $37.00
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 3145 n
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 3145 n
$64.18
Time Remaining: 25d 19h
Buy It Now for only: $64.18
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
$64.18
Time Remaining: 28d 18h 24m
Buy It Now for only: $64.18

Other On30 Scale train items recently purchased through this site:

On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
Hawthorne Village Silver Moon Express Leader of the Pack Box Car Track HO Scale
Hawthorne Village Silver Moon Express Pack of Friends Box Car Track HO Scale
Hawthorne Village Silver Moon Express Wolf Walk Box Car Railroad Track HO Scale
Hawthorne Village Silver Moon Express Wolf Ridge Box Car Railroad Track HO Scale
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 3145 n
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606