Buy.com offers the Kensington ClickSafe Keyed Notebook Lock 2-Pack, model no. K64661WW, for $47.99. This $15 mail-in rebate drops it to $32.99. With free shipping, that's the lowest total price we could find by $13. It features a tamper-proof lock and pivoting head; each lock attaches via the Kensington security slot (found in 99% of computers.) Rebate expires July 24.