Judging from posts here and a number of other boating related sites ( there are others?) , a lot of people have looked for them for a long time . They have been out of production by Aquameter for many years.
If you do not end up finding a used one, as a verrry long shot you might give lspeedo dot com /Lauderdale tachometer & Compass a buzz to ask if there is any chance they can repair your old one if you do already have one. They do claim they repair most all types of boat, car, or plane gauge but this is a little different.
FWIW, An alternative might be a Faria Instruments Dual-Engine Synchronizer needle gauge , model # 33105. Not digital like the Ensign , and I don't know whether Faria still sells them but I just saw a marine store web ad at $140 for the round dash gauge. Additionally, some dual engine/single gauge models of FloScans ( 7000 & 9000 ?) do have a sync indicator as a one of the many built-in features, the Lowrance LMF-400 offers sync feature, and there may be others as well. All those are passive indicators only like the Ensign.
But a much more elegant & pricier ($700=?) solution to consider is to install an active engine synchronizer like the AccuSync (by Sturdy Corp) or the venerable Glendinning systems. I have had the AccuSync for a long time and wouldn't be with out it . As the prop & engine loads vary in seas, it constantly auto-matches the rpm of the designated slave engine to the master whenever the switch is on and throttle levers are within ~ 15% of each other. Very definitely worth the cost on my gas MPI engines( & on the previous carbed engines) & I love it.
http://www.sturdycorp.com/gallery/detail.aspx?id=6630