You can't have looked very hard mate! There's thousands of cases on ebay, I picked up an old 24" server case for 30 quid including postage and packaging just last week!
A decent tip is to click on the price column and select cheapest price n postage first. This way you'll get the old, cheaper cases first.
Also I'd recommended buying a cabinet or a wardrobe for a stealth grow, pc cases really limit you and you wont harvest much at all. I'm just using mine to stealthily store male plants!
Safe