I do NOT think many companies will go out of their way to market stuff that was discontinued many years ago, and even back then, they did NOT sell well, I know we had shops back then, and most of the grip companies we dealt with then are now GONE>>>>>>>>
I have lots of OEM Speed Six grips and I believe I have some aftermarket (Pachmayr, etc.) Drop me a PM with what you are looking for (size, style, etc.) and I'll look through mine to see exactly what I have.
When I got my Six it had been rode hard and put away wet.... I re-finished it and did a real nice trigger job. Then I figured that a nice set of Stag grips would put a real nice finishing touch on the project. I also replaced the front sight with a Mepro night sight blade.
Watch E-bay. They often list OEM and aftermarket grips for a Speed Six.
If you find some the old Herrett Shooting stars can be modified into a fine grip for carrying. Back when I used these I cut them off at the bottom to within about 1/8" or less from the frame. rounded the edges and re-finished.
Here is whey they looked like along with my current grips that I got many years ago from Don Collins.