eBay is a good place to look. Go to their search page and type in skelly: http://pages.ebay.com/search/items/search.html
There is usually a nice selection at any time. Click on the link at the bottom of the results page to "Save this Search" and you can easily run it anytime or automatically send you email with Skelly that were recently added. Click on the "Completed Auctions" link to see what things have been selling for.
Also - Check the Events page at this site: http://www.oldgas.com/pp_swaps.htm
Nearly every petroliana swap meet that I have attended has had a selection of Skelly items. I just picked up this Skelly sign for my collection at the recent Columbus, Ohio swap meet.
Skelly often had 2 different messages on each side of a 2 sided sign. This sign says Tagoline Motor Oil on the other side.
Edit to update image URL
[This message has been edited by Oldgas (edited 03-24-2002).]