Any petshop that sells reptiles will carry them. Petsmart definitely has them, but they are expensive. Shop around. They are with the red heat lamps. Look on Ebay, though. Put in "ceramic reptile" or "reptile heat*". You can probably save quite a bit.
I got a 165 watt ceramic bulb on Ebay for $10.50 plus a couple bucks for shipping. It keeps my brooder, which is a 45 gal. rubbermaid tote, at 95. I love it, I can't wait to use it next month!