Modushop in yurp and parmetal in the graceland. There are others but these seems to be of good value. Check your local electronics shops, junkyard and tubeampstores. Uraltone sells some here in Finland that are ok and I use them to support local market

Thomann, canford.co.uk... just keep your eyes open, the Boxes are everywhere! (except Mouser doesn't have'm..)
Magic words for Google:
Rack chassis/case/housing
http://www.modushop.biz/
http://www.par-metal.com/
http://www.uraltone.com/
http://www.canford.co.uk/
http://www.thomann.de/