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'.

Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB
$46.99
Time Remaining: 13d 11h 58m
Buy It Now for only: $46.99
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
$64.95
Time Remaining: 23d 9h 52m
Buy It Now for only: $64.95

Other On30 Scale train items recently purchased through this site:

Thomas Kinkade Lamplight Express Observation Car14 09004 004Hawthorne Village
Heroic Observation Car Hawthorne Village Fire Fighters Heroes Express MIB