Panels for Penta were still available as of 6 months ago. In fact most parts for the Penta range were available with the exception of the display covers.
Panels for Beolab 3000/4500 and 5000 have not been available for some years. A third party was making replacement for this range but they didn't seem very popular, possibly because they were highly polished rather than the delicate brushed finish on the originals.
The originals for both ranges are held on with double sided tape which was also still available iirc!
Removal is a matter of brute force with a pair of pliers and a screwdriver or chisel.