Marco Rocks has some nice rocks.
Marco Rocks The finest aquarium rock available, base rock, live rock, reef rock, marco rock, reef tank saltwater fish, live corals, Marco rocks, Fiji live rock, Tonga Live rock
I bought some a few months ago to add to my existing structure. They have lots of holes in them so I took advantage of this feature to stack them using nylon threaded rod to connect them. I didn't need to drill any holes, they were already there! This will help to stabilize the rock formation. The last thing you want is a rock slide that could trap/kill your tank inhabitants, or worse, crack/shatter your glass.
Have fun with the aquascaping and use the $500 to buy something cool for your tank. Just make sure the structure is secure.