This peaked my curiousity. NOS ones are surprisingly expensive for what they are.
Lots of Corvette guys talk about building their own. It's a simple 4 component circuit if you're handy with a soldering iron and want to build your own in a small Altoids box to bolt to your firewall.
Taken from here: new tach filter schematic
If you have a nearby junkyard, it'd be simpler to go there and find probably just about any GM passenger vehicle from late 70's to mid 80's with a distributor and tach, cut it out, and splice it into your car.
Hope those random thoughts help!