I don't think the mounting bolt and washer is anything unique to the 9150 FD. Grab one of another Shimano FD and you should be good to go.

Here is the Shimano exploded view doc: http://si.shimano.com/...V-FD-R9150-4063A.pdf

It is hard to find Shimano parts online so you likely need to order thru a Shimano dealer. Might be easier to just check with an LBS to see if they have a bolt and radius washer in their parts bin.

A standard stainless metric bolt can be found at a hardware store, but inding a radius washer/spacer would be more difficult.

A quick search on eBay turned up this option: https://www.ebay.com/...0:g:DlEAAOSwLVZVtCLm

If you are interested in running a chain catcher such as K-Edge they come with a replacement mounting bolt and spacer/mount that take the place of the standard bolt and radius washer. The newer K-Edge Pro installs much easier as it has an additional bolt for adjusting ring clearance vs having to mount and adjust clearance using just the mount bolt.

Hope this helps!