You can check at places like Green Chapter, East Ocean or Fishy Business... they should have a selection of different themed rock types to choose from for aquascaping.
Rocks from landscaping or construction companies could also be used (and are usually much cheaper), but as with any hardscape materials, you have to wash them throughly to remove any residual chemicals or pesticides, and as a recommended precaution, it'll be best to put a sample of those rocks in a bucket of dechlorinated water for 1-2 weeks and test if they affect the water parameters significantly.
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