I love to send people to aquarium stores for supplies for larger enclosures. Especially if you have a really good local fish store that has aqua scaping supplies. They typically have larger rocks and pieces of wood that are already ready to go in an aquarium, so they should be safe for your snake. It's hard to find bigger pieces going to pet stores, and typically buying stuff this way it ends up being a little cheaper.
You can also check out arts and crafts stores, I got some great deals on fake plants that were way cheaper and better quality than what I was buying from the big box pet stores to do me decorating. You still wanna wash everything, get the dust and any chemicals off, and pay attention to what they're made of, but you can usually find some good stuff.
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