Tri and Ultra Tri Lux Differences/Source

What is the difference bewteen these two bulbs?  Is it a big difference?  I
bought a Tri-Lux bulb at a local pet store, but I want to start buying them
from a cheaper source.
The only mail order catalog I see carrying them is Mail Order Pet Shop and I
have heard they are not a good company to do business with.  Is there another
company that carries them?  Thank you for your help.  Jeff