Where can I find vintage hardware to mount a Raleigh 3 bolt chainring? I need hardware (3 short bolts) for my Raleigh Super Course 10 speed made in Nottingham England in 1973.

The bolts hold the front large chainring to the smaller chainring that the pedals are mounted to.   Can these bolts be purchased?
Based on the "three bolts"  you have something similar to this image, which appears to use three simple bolts, although I can't find what the threading is.


  TL;DR, there's an updated answer below all this waffle.


Sheldon's relevant page is at https://www.sheldonbrown.com/raleigh26.html and specifically fingers the Nottingham factory for making up their own standards, and did this right through to the 1990s.



If you have access to one of the bolts, then its possible to identify it using some basic machining tools. 


A Thread pitch gauge will tell you how many turns per inch (TPI) (aka the "pitch")
A micrometer or Vernier caliper will tell you the outer diameter.  It will probably be a straight thread - if the diameter changes along the length, then it could be a tapered thread
Eyeball the thread form - is it an even triangle (most likely) or something weird.


More info at https://www.classicfasteners.com.au/content/Identifying_threads.pdf and 
                
            

A couple of useful thread identication charts are 


http://almetal.nl/en/techinfo/gti/ticas.htm
https://www.portlandbolt.com/technical/thread-pitch-chart/
https://www.boltdepot.com/fastener-information/printable-tools/default.aspx


I'd just about guarantee its not a metric thread, but it could be Imperial,  UNF, UNEF, or something older and weirder like Whitworth.  Heck I even have some "BA2" bolts in my British Landrover from 1973, that standard dates from the days of steam boilers!

Still possible that Raleigh were completely alone and used their own standard bolts too - which may need to be made specially.  You could potentially retap the holes to a normal metric standard bolt too.

Finally if you don't have the bolt, all this is possible to measure using the threaded hole in the crank too.  Just a lot more fiddly.



If you do buy commercial bolts, try for stainless steel or high tensile (or both if you can) and use antisieze.  Try and avoid using the cheapest low-carbon chinese cheese bolts from the local discount store.



UPDATE


  I believe the 3 chainring bolts are 9/32" Outer Diameter and the thread is a 26 tpi whitworth thread.


From https://www.bikeforums.net/classic-vintage/703071-chainring-bolts-cottered-3-arm-crankset.html

There is also some discussion that a metric M7x1.0 thread will "fit"

The observant will object and point out that


Metric flanks have an angle of 60 degrees whereas Whitworth is 55 degrees, 
Metric pitch of 1.0 translates to 25.4 tpi while Raleigh's propriety threading is 26 tpi
and 9/32"= 0.28125 " = 7.14375 mm which is pretty close to a 7mm bolt.  


Add some thread locker to help stop it walking out, or you might have room for a jam-nut on the other side, maybe.