ebay has the 10mm ones in their marine and rv section.
Oh BTW, this is great simple mod. Jamie's looks great but I took the cheap route and had a plate made by someone local based on JamesCH's design. Total about $20 out of pocket.
This is the way the shifter should have been from the factory. If you are on the fence about this one, DO IT!
I went to McMaster Carr, its great living 10 minutes away from their warehouse, got 2 studs for $2.44 with tax. Then drove to Lowes and bought a 1/4" x 1 1/2" x 3' piece of steel for $8.xx. Went home and made it in about 20 minutes. Took two tries but hey, I have plenty of steel and an extra stud to make one for someone else!
I just want to thank everyone for the ingenuity in creating this design and making the possibility of DIY!