Have you checked SurplusShed? If they don't have any on their site, worth emailing them as I have read that the lucky people that are able to visit the store(USA) find things on the shelf that are not listed on the site.
They have a 79mm uncoated flat listed for $25(!!), but 1/10 wave:
http://www.surplusshed.com/pages/item/l10768.html
They also list some 1/20 wave flats but too small, worth emailing them.