I have these o-rings in stock.  I usually supply them with the heater core
(if someone just orders a core I supply it automatically since you are going
to need it to install the core and I hate to get calls or emails saying, "I
have this part but I can't install it because I need this other part.")
The o-ring itself is $2.50.  You can give me a call or order through the web
store and we can get it out today via Overnight shipping or 2nd or 3rd Day.
Whatever you need.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242