Underwater Galleries makes a fake rock that I have been happy with. Not sure what type of fish you plan on putting into your tank. Theirs are hollow so they serve well as caves , are light and don’t displace much water However they are not huge. Biggest are about half the size of a football. They work well for African cichlids.
Universal rocks also makes fake rocks but I have not purchased any of them so can’t speak for them.
All of the rocks below save the slate are examples.
+1 on the rockyard / landscape company suggestion. I bought approx. 200 lbs. of Texas Holey rock from local rockyards. Cost 12 cents to 25 cents per lb.