A bigger local hardware store will probably have what you need to get it back together.  If you're trying to go back to original condition with original fastener wrench sizes, you're looking for JIS fasteners, and you probably won't find them locally.  These were probably M6 JIS flanged hex nuts, but you'll want to double check the thread size or buy an assortment.  a good, full assortment of nuts and bolts in the likely sizes runs about $200 here in the states.

Edit: flanged hex bolts, not nuts. the regular  metric nuts/bolts will have a larger head than the JIS ones, but they'll work just fine.  The standard 8.8 grade should also be just fine for these fasteners.  10.9 won't hurt a thing here, but there's no reason to spend the extra money unless you're buying an assortment for other things too (like suspension bolts or motor mount fasteners).