As far as I know, McMillan will still build the rear grip target style stocks. I would give them a call to verify that though. The last six center grip stocks that I have ordered from them have been 5-6 month waits....
Consider going to Specialty Pistols Forums and put a WTB in the classifieds. -Somebody there is likely to have one they would part with.
I have a "unknown" brand rear-grip target stock for a XP with flat bottomed forend. Fiberglass stock.
I would sell this for $225 Shipped.
Emma Creek Gunworks out of Sedgwick, KS can make one for you (Wood).
Joel Russo, also makes laminated rear grip stocks.
Thanks guys. Thinking of doing a 6br pistol in the future. I called and emailed Kelbys sometime back about the stock they offered at one time but haven't heard back from them. I'll see what Mcmillan has to say.