I assume you are talking about the barrel nut, right? It slips into the side of the riser and the limb bolt screws into it, right? Call Martin at 1-800-541-8902. When prompted push 2 (?) for warranty dept. If you have to leave a message and most likely you'll get Joel. He'll take care of you. Or you should be able to buy one at any Fastenal store. Maybe even the specialty fasteners at Home Depot or Lowes (hit or miss).
I wouldn't waste time with emails to Martin. I know of many people who have tried with no response.
Barrel Nut..........Part# BP-28
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver