Your Shimano dealer LBS might be more help than someone in the forum. Do you have a bike shop close to you?

Looks like there are several screws that press against the frame for the di2 FD.
Go to this page (will pop open as a pdf, make sure your pop up blockers are off):

si.shimano.com/php/download.php?file=pdf/ev/EV-FD-6870-3640.pdf

Figure out which screw(s) you need. Use the part number(s) to search for it. Order online or at your LBS. I am pretty sure they are avail as small parts.

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Sounds like he needs the support bolt, item number 3. These guys have it. I am pretty sure dura ace and ultegra use the same bolt. Comes with extra support plates too. Available here.

http://www.bikesomewhere.com/...flat-type-y5nf98030/

Note for those unfamiliar with the fastener isle at a really good hardware store - the "M4 x 8" etc on that parts diagram for each screw is the size. It turns out that many (but not all) bolts and fasteners on any bike are standard sizes and types of fasteners that more often than not can be found at any really good hardware store. I'd avoid using hardware bought fasteners for critical items like stem bolts (those often are specialty shaped items anyway) but for things that are not holding your weight, the local hardware store is a good option to keep in mind for short term "I need it right now" fixes [edit: to get you by until you can get your hands on the real part].

This particular one is a set screw style head. If you get a regular m4 with a socket head, it won't work in this application.