When I first got into ponding, I found a preformed liner for cheap at a rummage sale and bought it. Seemed like a good deal, but I ended up never using it, so it wasn't that great of a deal, I guess. I went with a flexible liner instead.
I don't go to rummage sales often anymore. My grandma's subdivision used to have a big one every year, the first Saturday in June. It was a lot of fun going to when I was a kid. The whole family would show up and walk around. My aunt, who lives in the same subdivision, would have a rummage sale every year and we'd always visit the night before to hang out and help set up. It was always a lot of fun. Unfortunately, over the past 10 years or so, people stopped participating. It went from at least 30 houses participating to around 4 or 5. They cancelled it this year for the first time ever. Coincidentally, it would actually be tomorrow if they still had it.