They're just a bit rarer than pull tabs. It's a highway sign which (unless the local chapter is no longer around) was meant to stay put, and probably was removed by some kid who thought it would be a swell trophy. Although, presumably they might replace them if they were too worn, but I see worse ones still roadside, so...
Anyway, they do show up on eBay, though I'm not sure they should, and recent selling prices were 9.99 and $25 for one in freshly stolen condition.