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

BEMO 2254 Gbv FREIGHT STOCK CAR Gterwagen NEW BOXED mc
BEMO 2254 Gbv FREIGHT STOCK CAR Gterwagen NEW BOXED mc
$31.35
Time Remaining: 2d 8h 48m
Buy It Now for only: $31.35
HOn3 CUSTOM BUILT 24 TRUS ROD WOOD STOCK CAR KDs STEEL WHEELS PAINTEDb
HOn3 CUSTOM BUILT 24 TRUS ROD WOOD STOCK CAR KDs STEEL WHEELS PAINTEDb
$29.95
Time Remaining: 28d 22h 24m
Buy It Now for only: $29.95

Other On30 Scale train items recently purchased through this site:

HOn3 CUSTOM BUILT 24 TRUS ROD WOOD STOCK CAR KDs STEEL WHEELS PAINTEDb