Russ, For past few months I have read most of the literature about the 4.99, 4.107 and 4.108 trying to become my own diesel mechanic. Sorry I can not give you the exact answer, but here goes the best info I have.
From the Perkins M4108 manual I found and attached a pdf file specifying the gauge. Unfortunately the page with part numbers that provides the gauge part number says "Other Reference numbers not applicable"
From the Perkins Marine Engine Handbook I found the attached pdf snippet specifying the gauge pressure readings.
I have heard from BMW's (boat marina workers) that they have used truck or auto oil pressure gauges. Not my suggestion but in a pinch it may work. Think I would try one of the Perkins distributors. Note that the Perkins engine has been used in the trucking industry to power refrigeration systems and some small Hyster lifts. So you may find a diesel shop which has what you need and at a "non marine" price. Since the gauge's salt water exposure may be limited (unless you plan to fill the engine room with Salt Water) a non "marine specified gauge" be an answer to your challenge.