Yes, you absolutely can buy just the legs of a street bench and build your own wooden seat. In fact, it is a pretty common project for people who have an old rusted bench frame with rotten slats, or who simply want a custom look without paying for a full pre-made bench. The legs and side supports are usually sold separately as a "bench frame kit" or "bench ends only," and they come in a few standard materials like cast iron, galvanized steel, or powder-coated steel. You can find them at hardware stores, online marketplaces like Amazon or eBay, and specialty outdoor furniture suppliers. Just search for "bench legs only" or "bench frame without seat" and you will see plenty of options.
When you buy just the legs, there are a couple of things to pay attention to. First, check the width. Standard park bench frames are usually built for a seat that is between 48 and 60 inches long. If you are replacing an existing seat, measure the old frame from center to center of the mounting holes. If you are starting from scratch, decide how long you want the bench to be and make sure the frame you buy matches that spacing. Second, look at how the wooden slats attach. Most bench legs have pre-drilled holes or brackets on the top rails. Some use carriage bolts that go through the wood from the top, while others have threaded inserts underneath. Knowing this helps you decide whether you need to drill your own holes or just line up with existing ones.
For the wooden seat itself, you want to pick a rot-resistant wood if the bench will live outside. Cedar, pressure-treated pine, teak, and white oak are all good choices. You will typically need three or four slats, each about 2 inches thick and 3 to 4 inches wide, with small gaps between them for water drainage. Measure the distance between the two side frames, subtract about 1/8 inch on each side for expansion, and cut the slats to that length. Then lay out the slats evenly across the top rails, mark the bolt holes with a pencil, and drill through the wood. Use stainless steel or galvanized bolts, washers, and nuts to prevent rust. If the frame does not have pre-drilled holes, you can just drill through the metal top rail as well, but this is less common.
One thing to keep in mind: buying just the legs is not always cheaper than buying a whole bench, especially when you factor in the cost of quality wood and hardware. But the advantage is that you get exactly the seat material, color, and width you want, and you can replace just the slats again in the future without throwing away the whole frame. It also lets you reuse an old set of cast iron legs that still have life in them.
So to answer your question directly: yes, you can buy just the legs of a street bench and add your own wooden seat. It is a straightforward weekend project as long as you measure carefully, choose outdoor-grade lumber, and use rust-resistant fasteners. Start by picking a bench frame kit that matches the seat length you want, then build the slats to fit. You will end up with a sturdy, custom bench that looks like it came straight from the park.