you could do what i did and search around until you getreally flat pieces of rock.
then you can order rocks from reeferrocks and they'll give you whatever kinda shapes you want.
check this out:
my tank is set up now and it is FULL of shelves!!!!!!!!! I did all of mine just by stacking the rocks and useing shelf pieces to make shelves poke out.
I'll try to post some pics of my tank tomorrow if I can remember..... it's only a week old though so