Head to the landfill in the electronics drop off point. At mine, every single trip I go, they have random beige/black ATX and AT cases laying all over the place. It is usually best to go before 4pm, as a lot of scrappers usually come after they get off of work and pick up computers. Also check the rules of the place so you don't get in trouble for taking anything that is not allowed to be taken. At one of mine near my house it is basically a free-for-all, and another one about 10 miles away they don't allow things to be taken.
You may also find electronics recycling yards near you that you didn't know about. A co-worker recently told me about one that is about 20 miles away from me that has boatloads of stuff they just send off to China for "recycling", but they also sell it by the pound to anyone who wants to look through it. It's extremely cheap -- they have everything from desktops to printers, to servers, to 42U rackmount cabinets, etc. At that place I haven't had much luck getting anything good on both trips because eagle eye scrappers/resellers snatch all the good stuff as it gets unloaded, but my buddy got all sorts of awesome stuff the last time he went. (multisync CRTs, beige AT cases with MHz displays, several ISA sound blaster cards, a few 386 socketed motherboards, and VLB 486 motherboards)