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 KINKADES CHRISTMAS EXPRESS ISSUE  7 CHRISTMAS CHEER OBSERVATION CAR
THOMAS KINKADES CHRISTMAS EXPRESS ISSUE 7 CHRISTMAS CHEER OBSERVATION CAR
$34.95
Time Remaining: 9d 11h 59m
Buy It Now for only: $34.95
ENHANCED BACHMANN On30 Painter of Light Observation Car
ENHANCED BACHMANN On30 Painter of Light Observation Car
$110.00
Time Remaining: 27d 12h 49m
Buy It Now for only: $110.00

Other On30 Scale train items recently purchased through this site:

ENHANCED BACHMANN On30 Painter of Light Observation Car
THOMAS KINKADES CHRISTMAS EXPRESS ISSUE 7 CHRISTMAS CHEER OBSERVATION CAR