My two DT300i's and the previous Yager 200i all used a metric machine screw with either an 8 or 10 mm hex head and a captive washer. Lowe's, O'reilly's, Advantage Auto have all kinds of metrics but probably not captive washers. Already suggested by Neil to take the orphan to one or all of these places and get four the same. Now, you can get fancy and make your own captive washers if you want. Just say the word and I'll tell you how.
I seriously doubt any remaining dealer is gonna have these but you might get lucky....