There’s a Builders located just a 10-minute walk from the waterfront at 17 Waterfront Drive. There’s a much larger store located in George.
This is a large hardware store where you can find tools, hardware, etc. Note that the Knysna store doesn’t carry stainless steel fasteners (go to Tool and Trade for that). But they have a good selection of tools. You can shop online and have them ship items that they don’t stock locally to the store.