Look on ebay for industrial filters.
You may need larger, but it's this sort of thing: http://www.ebay.co.uk/itm/2X-Carbon-...3D291709689561
The stuff was called 'Declon' and was first popularised for speakers by JBL I believe, who made the front grilles for the L100 Century model from it around 1970.
Main system: MMs/ADCs/Deccas/Low output MC's/various SUT's/various arms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT/Swiss Idler jobs. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC, stepped attenuator or valve pre-amp/Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big and smaller dual concentric speakers and full range speakers, amongst others. And too much more to list!