Try a different hardware store... I had to do the same thing today for my blaser r93's receiver. I had to shorten a little bit but it was alot cheaper than ordering online, paying shipping, etc for an 80 cent bolt.
They are M6 x 1 x 50mm Metric Socket Head Cap Screws (Full Thread). I had no luck finding anyone who sold them individually in black locally. I just bought 100 extended M6 x 1 x 70mm bolts from MSC online for a new one piece spacer I made. The 30mm stock spacer (three 10mm spacers) weren't enough for my correct LOP. Rather than ordering in some more spacers I had some black nylon lying around at work and decided to just machine a one piece 50mm spacer. I have milled it down to 40mm which is perfect now. The bolts I have wont work as stock replacements as they have a 1.5 inch shoulder on them. If you are planning on adding an extra plate or two I could send you some of these. They will work 40mm-60mm of spacers. The 40mm spacer will just need to have some length cut off to keep from bottoming out in end of the stock.
opposite here.. impossible to find an imperial fastener.. took me 4 hrs worth of driving around to find some bolts the same thread as Rem 700 action screws.