Lots of live rock questions today
One place you could check would be www.liverocks.com
they are one of our sponsors and if your really interested you can post in their sponsor forum. They offer FL aquaculture rock.
For Fiji rock you could look at www.liveaquaria.com
or just about any other decent size online retailer. 99% of the fiji rock comes from the same source (Walt Smith International).
Well for inverts that you can get away with on a 90 gal
tank with 3 regular florecent bulbs I would have to say hermits and shrimp. I really don tknow many corals that you couldkeep under such low light conditions. I dont even think mushrooms would fair very well.
How much rock to get? Well if you intend on using the rock as a filter then loook for at least 1lb per gal
of rock and if you can afford it closer to 1.5lbs per gal
of tank capacity.
So in your case that would be 60-105lbs of live rock.