Amazon has a few:
http://www.amazon.com/gp/offer-listing/B00...p;condition=all
The prices are quite outrageous, though.
Other than that... only eBay:
http://video-games.search.ebay.com/arcanum...R40QQsacatZ1249
Two auctions currently running just below $30 on US eBay.
Also, another for 17.99 on the UK eBay:
http://cgi.ebay.co.uk/ARCANUM-of-Steamwork...4QQcmdZViewItem
I've ran through our list of online stores AND several different angles of Googling for the thing. Seems like it's one of those unfortunate low-supply/high-demand cases where the prices start skyrocketing after a while.
And by the way...
expansion? I didn't hear about Arcanum ever having an official expansion released, so please explain what did you mean...