Being that it is a Raleigh of mid '70's vintage, the bolt was likely whitworth threaded, and therefore, will be nearly impossible to find a replacement for. You could check a classic bike forum, especially one for old Raleighs (they were the only manufacturer using whitworth standard threading since the early 50's), or you could have a machinist make you one. An engine lathe could be used to make one fairly quickly, but expect to have to pay dearly for it.
Another alternative would be to drill out the threads, cut new threads with a die, and use a larger, common bolt.