I recently discovered this great UK site with (I'm guessing) 100's of printable artwork/images.

The artwork varies in size but are all easily downloaded in PDF format.

The Olympic Medals to Print is very topical currently and the artwork might make a great background for a 58mm pin badge.

http://www.activityvillage.co.uk/olympic-medals-to-print

I strongly urge you to have a browse through the site, there are lots of other great ideas there