The Antec sx1030 and sx1040 lock both the front and side panel, the 830 and 840 just the side panel. These will prevent casual tampering, not serious B&E. Also sold under the Enermax, Chieftec, Chenming and Uneec labels.
Many boxes have a padlock hasp for the side cover. This box has a lockable front, but the power&reset buttons are exposed- I have no experience with it-
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Some fileserver boxes are also lockable, if ridiculously expensive. I suspect that some rackmount stuff has good locks & hardware, but no experience with it.