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'.

ENHANCED BACHMANN On30 Painter of Light Observation Car
ENHANCED BACHMANN On30 Painter of Light Observation Car
$110.00
Time Remaining: 13h 18m
Buy It Now for only: $110.00
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
$64.95
Time Remaining: 9d 5h 18m
Buy It Now for only: $64.95
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express
$46.99
Time Remaining: 29d 7h 24m
Buy It Now for only: $46.99

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