Yep, check the keysight site in case anything in production still uses the part. (Get the part number from the docs.) I just bought some key caps for a 3458a (which is still in production) - but, for instance, the same ones are used on the 3245a (which is not). Easy and <$10. Else find a CAD person, bribe with beer (the CAD should take < 1/2 hour) and send the stl file to shapeways or imaterialise. You might want to order the part in a couple of different materials to see which one matches the best. However, small parts are cheap, say <$5 in one off.
Alan