evga 6600GT BIOS's here, as well as many others. Hope you know what revision yours is.
if for whatever reason that page doesn't load, just go to the main page www.mvktech.net,
then downloads on the left, then nvidia, then BIOS, or whatever the order is. I'm sure you can find it.
Why do you want a BIOS update anyway? If your card is working you really shouldn't mess with it.
If you're wanting to enable temp monitoring (which when I had a 6600GT it didn't come standard) then go here:
Scroll down until you see Symbios's post and "How to enable temperature monitoring on Nvidia GeForce 6600 cards"