Do you know what kind of light you want? All T5, MH, or a combo unit?
I like reefgeek.com for T5 lighting fixtures, either the ATI sunpower or ATI powermodule are widely regarded as some of the best, and reefgeek has good prices with any bulb combo you might like.
There are alot more choices in MH that are good, alot depending on the look you want, vs the light output you are trying to acheive. Do you want a single 48 unit or are pendents an option, if you are really focused on the highest light output over looks it changes things.