Martin Are you certain on the 5/8" x 3/16" slot? A 3/4" wide slot is the standard size, and you should be able to pick-up a miter at most homecenter/hardware stores. If not, do some searching in your yellow pages for an industrial tool supplier (like Delta, Porter Cable, Ryobi, etc.). If the slot is not the standard size, perhaps having it machined to the correct size would be appropriate.
Here's a couple links that might help also... http://www.toolpeddler.com/index.htm http://store.yahoo.com/toolsplus/woow1323.html
Or try these phone numbers... Delta Machinery Corp.(800) 438-2486. Dewalt Industrial Tool Corp. (800) 433-9258. Makita USA (714) 522-8088. Milwaukee Electric Tool Corp. (414) 781-3600. Porter-Cable Corp. (901) 668-8600.