Base rock

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hi everyone, I am new to the site and to the hobby. I have done alot of research and know what i am getting into. I am having a hard time finding base rock for the tank. I am looking for about 50 #s to go with the live rock. evey where i have checked is always out of base rock. any reccomendations?
by the way the tank is a custom built 131 gallon with a 25 gallon sump.