First question: How secure do you want it?
If this is only to keep people honest, and not to legitimately stop a determined thief, just go with one of the cheap ones on eBay (search "laptop lock" or "kensington lock"). I have one I picked up for $1.50 on there, and it works just fine, and feels plenty secure.
However, If you are looking for something beefier, possibly with a warranty of some sort, you may have to get something name-branded, which usually runs you ~$45+.
All that being said, any of these will not stop someone determined enough. These all ultimately just prevent the casual thief. If someone wants your stuff, they will take it, if they are determined / resourceful enough.
[EDIT]
Just to be sure you know, since you seem to not be a lock expert or anything, you will be looking for a lock that will attach to your iMac's "kensington lock slot". It is a little slot next to all the ports on back.
Most of these locks are cables that have a loop on the end, that you wrap around the leg of a table, etc, and then put the lock into the slot on your iMac.
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