The bolt is M8x15, 1.25 thread. I just pulled one off my '99 3rz; it's still warm in my hand.

The factory bolt has a flat washer that can't be separated - the washer was installed on the bare bolt and then the threads were rolled on - so just get a 15 or 16mm bolt and add your own washer. It's not a critical bolt so any old M8 bolt will do.

Revision: I found the part number: 91642-C0814. (from page 2 of the Wiring Clamp diagrams) So its length is 14mm. I had measured the bolt I took off in the standard way one measures a bolt's length, from the underside if the head. Apparently, Toyota measures bolts with affixed washers from the underside of that washer.

Lamentably, you may find that the aftermarket bolt has 13mm flats, whereas the factory item has 12.

As an aside, I've never had any luck finding M10x1.25 bolts in hardware stores or Loews or Home Depot. There are, in my experience, very few instances of M10x1.5 thread on Japanese-made vehicles, yet that's what's commonly retailed in the US. Ditto for M12.

DIN Metric Thread Pitch Chart (Coarse, Fine & Extra Fine) | The Fastener Resource Center