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

Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
$64.95
Time Remaining: 5d 10h 17m
Buy It Now for only: $64.95
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB
$46.99
Time Remaining: 26d 12h 24m
Buy It Now for only: $46.99
ENHANCED BACHMANN On30 Painter of Light Observation Car
ENHANCED BACHMANN On30 Painter of Light Observation Car
$110.00
Time Remaining: 27d 18h 17m
Buy It Now for only: $110.00

Other On30 Scale train items recently purchased through this site:

Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB
ENHANCED BACHMANN On30 Painter of Light Observation Car
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village