You should still be able to obtain the ceramic type from most auto parts stores.
My 73 Bug uses the ceramic fuses as well, and I have no problem procuring them.
If your local auto parts stores don't have them, check the import car shops...they should keep an ample supply...
The ceramic fuses use either a copper or zinc metal, and over time they both can discolor...replace those that look suspect.
It they indeed look "burnt" make sure the fuse receptacle has the proper amperage fuse connected. Otherwise, it is probably cosmetic, and they do an excellent job of "blowing" anytime I screw something up electrically...
BTW, the BUSS type fuses I see are primarily used as "in-line" fuses and not part of a terminal block...