I don't think your'e looking hard enough. Quick glance at the BMW site showed 3 convertible 6-speeds w/ red interior just in the first couple pages, of 12 pages total.
http://nitra.bmwcpodealer.com/cpo/Ve...-NY/1904532573
http://nitra.bmwcpodealer.com/cpo/Ve...-AZ/1895647863
http://nitra.bmwcpodealer.com/cpo/Ve...-AZ/1913213083
Granted they could be sold, or mislabeled, but it seems to me there's more than just 1 to choose from.
Also, FYI, if you're really seriously looking, you need to check *all* ads on the BMW site, ebay, and etc., because a ton of 6-speeds get labeled as automatics and vice-versa. You can't trust what's written in the ads, you must look at the pics. It's highly annoying!