A few dealers can help, as well as some gunsmiths. Trouble is, You need to know what length you need for proper headspace, and that's a bit of a trial and error thing. If you have a headspace gauge, and with you are closing on the "no go" gauge, use a caliper, measure how long your bolt head is, and you know you need one which is slightly longer. You then have to find one of the proper length.
But even then, you're not out of the woods. Sometimes with a spare bolt head, it won't screw properly to "12 o'clock". You may screw the new bolt head in, and find that it stops a few degrees just shy of 12 o'clock. Or, sometimes, it overturns PAST 12 o'clock by too much of a margin (I think the safe zone is below 20 degrees), and that's not good either.
Best is to stop by someone's shop where there are a few bucket-full of spare bolt heads
Lou