Yeah, I'm going to back-up AHBD here, please do not buy a regular compact or coil
UVB bulb that will fit into the fixture you have now, without a strong, long UVB tube inside a long tube fixture that has a reflector inside it, she will become seriously ill and eventually not be able to move, walk, she'll lose her appetite and sleep all day, and eventually die.
There are 2 strengths of the long UVB tubes: The much weaker T8 strength tubes, which is what you will find at PetSmart, and then the much stronger T5 tubes. Also, YOU MUST BUY A 10% UVB TUBE, OR A 10.0 UVB TUBE, NOT A 5% UVB TUBE, OR A 5.0, AS THEY ARE FOR TROPICAL REPTILES.
Please avoid the brands All Living Things (Petsmart's house brand), Zilla, and ReptileOne, and all 3 are extremely weak and all 3 seem to emit harmful light rays that cause eye damage, blindness, and neurological damage.
You want either the brand Reptisun 10.0 or the brand Arcadia. Reptisun 10.0 is what you'll find at Petco or Petsmart, but they usually only have the weaker T8 strength, or they'll have the stronger T5HO strength in a package deal with a hood, but it's only a 5.0 and not a 10.0 tube.
You can buy a 22" Reptisun 10.0 T5HO UVB tube for around $24 on Amazon.com shipped, and then a 24" T5-rated tube fixture with a reflector for about $28 shipped. Or the Fosters&Smith links that AHBD provided. That's definitely the way you want to go.
You'll also save money by buying the stronger T5 UVB tube, as the T5 tubes only need replaced once a year, while the much weaker T8 tubes much be replaced every 6 months. Also, the weaker T8 tubes have to be strapped to the underside of the mesh lid to the tank, they cannot sit on top of the mesh lid, as they are too weak to still emit adequate
UVB light to you dragon on the other side of the mesh. Also, the T8 tubes must be at least within 6" of the dragon, while the T5 tubes only have to be within about 10-11" from your dragon, so that gives you a bit more leeway if your tank is tall.