M42 nuts!! You'll also be needing a 65mm spanner as well then
We use Andrews Fasteners in Leeds. Was in their warehouse just the other day, and noticed they keep stock of stuff up to 50-odd mm dia. They have racks of the stuff. Sure they would be able to help you out.
M42 are expensive, this is now almost a non-standard size. The cost difference between a M36 x 250 bolts, and an M42 x 250 bolt could be as much as 4 times. Therefore, I would have thought there is a similar cost differential with the nuts.
Is this what you want? http://www.fastfixdirect.co.uk/code...egoryID=289&fType=Fasteners&search=Show+Items