You are in a perfect business! Have you looked at shipping bags from the states? Know anyone going out there who could bring some back in luggage? You could ask dez on RR he has built a few, he's currently doing his w114 merc.
Minilight list a 13x5.5 on their website. http://minilite.co.uk/products.htm
I bet there are a stack of OEM wheels that are that size from Renaults and and others.