You will be extremely lucky if you find any new Smiths Commando instruments.
Also, there were a few different models of MPH/KPH Smiths speedos and tachos used over the production period of the Norton Commando (1969-75) some Commandos were manufactured '76-77 and as far as I know those all had Smiths instruments. As at least two different styles of Veglia instruments in both MPH/KPH speedo types were also fitted from 1973-75, in addition to the Smiths instruments.
And presumably you will need a KPH version speedo?
So buying used replacements will depend on whether you want exact replacement "Green Globe" (early type Commando instruments were known as "Green Spot", or "Green Blob") Norton Smiths instruments or not? If not, then used Smiths Triumph style instruments are more commonly available. There are pattern (Triumph style) instruments available that are not too expensive, but I would suggest you have your original instruments professionally repaired, but I have no personal recommendation, but here's one that has been recommended by others:
http://www.speedorepairs.co.uk/