Keith is correct - replacement grilles for all avant variants are still available - however, I would maybe suggest you try washing the ones you have.
Unless the wear you mention includes actual holes, you should find they come up like new. The typical issue is all the speaker drive unit outlines, and some fixation holes, become visible as "shadows". This is dust in the grille cloth.
Take the grille off, then using lukewarm soapy ( ideally a fabric cleaner ) water, sponge them thoroughly. Try not to get them too warm & wet near the edges, in case the fabric glue fails.
Let them dry overnight- you should have nice, new looking grilles. 
Bang & Olufsen of King Street - Manchester,UK. SKYPE - beokingstreet