DWM, Uberti is still making the birds head grip I believe in the model P (Taylor, Cimeron, EMF, and others). There is also a Model P JR (Stallion) available from them as well. Has the same frame size as the colt Lightning and Thunderer (3/4 size). Not NCOWS legal, but can be used by SASS as a side match Pocket Pistol, since it is a single action.
There is not any actual documentation that a Model P was ever equiped with the BH grip from the factory prior to the turn of the century, but there may have been a Gun Smith out there willing to do it.
I would Imagine that you can still get a Ruger Single Six in the birds head grip, but I am not sure if they are in 38, maybe, only in the 32/20. Might just be worth a try going to their web sites. If you do not find what you need you can always go to VTI replica parts and order the back-strap and the birds head grips. Probably get into a complete set-up for less than $125.00. I did the opposite for a pinkerton grip that I had on an AWA Mod P.
A word of caution, if you want a grip and backstrap that fit with out some sanding filing, then you should probably go the extra cost and get the fitted varriety as a pair.
Vaya con Dios