Most pet shops that sell herp supplies, Amazon.com, LLLreptile.com, most other online venues that sell reptile supplies, reptile shows and expos, etc...
The best one is the mercury vapor bulb that has UVB/UVA. Gives, heat, light and the UVB/a . Also the tube flourescent. Yes as stated most any pet store or on line you can usually find them the cheapest.