There are a ton of options with different locking mechanisms. Kensington is the most repeated brand name, and they sell from simple to more bulky locks at places like Target, Walmart, Office Depot, etc.
I personally like the keyed ones better than the combinations because I used to always take my combo lock off and forget to "mix" the numbers before I walked away and anyone could of come by and seen my combination to steal my computer later on.
The Kensington Clicksafe Keyed Lock is 37$ and change at Newegg right now, and there are other locks for a lot less.