1991 Chevrolet gauge cluster

do a search for gauge repair and pick one close by, tthere are too many to list.

Klutch made the suggestion I would have. But I forgot the company...
I know of a shop in Virginia called Black Wolf that does restorations and might be able to help. AnotherVA shop is White Post and no idea about the black/white thing.

look these guys up, mail yours in and they rebuild it. I've had newer model GM products rebuilt locally, the fuel gauge feature fails. Also there are parts dealers that specialize in specific model pick ups etc. I purchased bunch of stuff for my K-5 Blazers .

MartynAuto
in Westfield, Massachusett

this came up in the first search I did, poke around using different key words. Also look for the motor head shop in your town and ask the hot rodders, they would know. Anyone that restores old cars is a valuable information source.

You could try these people.
Colaw RV Salvage
Your rig would normally be on a G-30 van. Many auto salvage yards may have G series vans.
Could check with local speedometer show to see if they have any idea of what is wrong. Could be cable or speed module. There is also a module mounted on top of radiator for converting some signals.