Since it appears you've elected to try African Cichlids, real limestone rocks work really well.
My tanks are all decorated with Texas Holey Rock / Limestone. Lava rock is good too except for the sharp edges. Both type rock have natural holes in it that cichlids like. Rounded river rocks also look good, but no holes. They need to be stacked in order to make hiding spots.
The cheapest place with the best selection to buy rocks is a local rock yard or landscape company. Your LFS
will likely be much more expensive.
Your 125 G. Is going to take a lot of rock. My 120 G. has over 100 lbs of limestone rock (THR) Cost me about 25 cents a lb. From the rock yard. My LFS
sells (THR) for over $4.00 a lb.