Try your local hardware store, I got ours at Home Depot, IIRC in the tool corral. You may find them at pet or kitchen stores, but probably for more than at a hardware store.
Reptile Basics has a nice article about temp guns and what to look for:
http://www.reptilebasics.com/tempgun-faq
The key is to look at what the temperature/error range is and the effective distance - so one that is (pulling numbers out of the air!) +/- 2 degrees is better than one that's +/- 10 degrees; and one with a useful range of only 4-6" away is less useful than one with a range of 6-36".
And remember that if you try using one through glass/clear plastic you'll be getting the temperature of the side nearest to you.
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