Micro Bed's gone, but Bisonite is still available. It should not be confused with the polyurethane product by the same name. It's the one made by Saturn Products
mentioned here. Brown color.
But if you are filling a hole in a grip panel, the question I have is are you going to have to re-drill into the filler material as part of correcting the hole location (is the hole location now off just a little bit)? If that's the case, you really want the filler to have the same hardness as the wood as closely as you can manage it, or the drill will tend to grab and walk toward the softer material an not stay on location (though using a small end cutting end mill rather than a drill could work).
Two ways to approach that come to mind: One is to obtain a dowel of the same wood and drill and glue it into place. If you get a plug cutter that's small enough you could even wind up with grain you could align to match the rest. The other is to get a clear epoxy and a bag of brown phenolic micro-balloon filler from Woodcraft and mix that in different proportions until you get a matching hardness. If you can't get that soft enough, you might need to go to a water thin epoxy like Rot-Fix.