My center allen bolt is stripped also. I now use a larger allen wrench (1/4) to slide the center bolt. But I will be replacing it this winter. Like the post said call Jay Marks Toyota ask to speak with Jeff Watson 1-800-327-2087 for all your factory parts needs.
You need to take off the headlining of the Targa roof and replace the whole center locking section that includes the stripped bolt. I paid (in Europe) around $60 for this part.