Everything I've heard and been told says you should have at least 1 lb of live rock, preferably 1.5 lbs, per 1 gallon of water. So, at least another 20 lbs. I know, it gets pricey, but you really don't want to skimp on the live rock. It's super important for filtration.
I bought all my live rock at my Local Pet Store. But shop around. Driving an extra five minutes to a different petco yielded me a gorgeous chunk of LR
with several types of algae, micro brittle stars, and even a couple tiny mussels. The tank really exploded into life (as much as a tank of just sand, water, and rock can) when I put that in.
You should also look into any local aquarium hobbyist groups, as they can give you better local information.
Again, get at least 20 lbs, preferably more like 50 lbs of Live Rock to add to the base rock and let the tank cycle completely.