Depending on where you live, most computer fairs usually have stalls selling them by the boxfull. Near here (Manchester) the big weekly fair at Bowlers has a stall selling nothing but cases in just about every combination you can think of.
Roadgiant and gOsip....good idea but in this computer backwater of Plymouth , the nearest computer fair is, I think, Bristol which is 120 miles away. If anyone knows of one nearer to Plymouth please let me know. Pooke100.. I visited my local branch of Tesco today and the only empty cases were priced at £2.47 for 10 in number ie 24.7P each.