I have just had my gauges restored or replaced, mostly using core gauges of the same type bought cheaply on ebay.
As we know, the British motor industry worked in a particular, efficient way (yes really) when it came to ancillary parts. Pretty much any part you need (with a few small exceptions) was used in something else, in a slightly different form. So far, I have found:
Speedo: Sunbeam Rapier (correct casing, style of dial, needle, trip reset etc. - would need recalibrating and new dial printing)
Tachometer: MGA/early MGB, with the ignition light in the face (becomes high beam in a Cobra - again, correct casing, style of dial, scallop for telltale, needle - would need reprinting and recalibrating, see below, or (better) conversion to RVI type electronic, using dummy cable from dynamo)
Fuel gauge: loads of cars, but early MGB or Frogeye etc. (would need reprinting)
The tricky one is the temp gauge as few cars used the original 30-100 range and it is difficult to change the internals on that one.
These were all sent to Tim at Speeodgraph Richfield in the UK, who will restore and repair as necessary and reprint the dial to whatever you want, including part numbers. Another possible problem can be the tacho, as the correct gearbox for the cable drive is readily available but only as the original 4:1 ratio. The culprit is the dynamo pulley, which was unique to the Cobra and very, very large diameter. Unless you have the correct one your tacho will not read correctly.
Note - the replica gauges readily available all use a wildly incorrect needle style, which to me sticks out like a sore thumb.