For big rocks visit a garden centre. Homebase for example stocks huge green granite boulders that are safe in fish tanks and cost about £3 each. That's very expensive for rocks, but cheaper than at most pet stores, and more convenient that buying them in bulk from an ornamental stone distributor. Some slate quarries allow customers to drive up and collect as much as they want for a certain price, and that can be a good deal too. Note that collecting from the beach may or may not be legal, depending on whether the rocks there are shore defences.
Cheers, Neale