Herters was an importer headquartered in Minnesota from the early 1960's to 1979. I don't know of any way to find the actual manufacturer, but it is probably a close copy of some Smith & Wesson model, so a gunsmith could look through his spare parts bin and maybe come out with a pair that would (or he could modify to) fit.
J.P. Sauer was making some handguns for Herter back in the 60s. I have Herter 44Mag SA made by Sauer.
Spacegold
Herter's sold various maker's products with their name on them.
It is my understanding that Herters only made rifles in the mid to late 60s and were sold mainly via mail order direct from them with a few stores selling as well. The Gun Control Act of 1968 pretty much marked the end of Herters rifles.
$100-$125
5-50 usd
50-300 usd
50-325 usd
You need a gunsmith
100-400 or so
100-450
Depending on condition, $150-$200.
Do a Google search on Herter's Parts. You'll get hits.
You need the services of a good gunsmith or machinist