That's an excellent and very practical question! The short answer is: not really, there isn't one universal "standard size" that fits all urban trash cans. While you can buy liners almost anywhere, finding the correct fit requires a bit of knowledge.
Public trash cans in cities and parks vary greatly in design and dimensions. A small sidewalk bin differs from a large rectangular park receptacle or a round barrel-style can. Therefore, their liner needs are different. Manufacturers of these public bins often sell compatible liners specifically designed for their models to ensure a secure fit that won't slip down.
However, many municipal and commercial facilities use liners based on common gallon capacities (like 30, 33, 55 gallons) or specific dimensions (width x length x gusset). You can often find these at janitorial supply stores, big-box retailers, or online. The key is to measure your specific bin—its height, width, and depth—before purchasing. Look for bags labeled with dimensions, not just gallon capacity, for the best fit.
So, while you *can* buy liners anywhere, your success depends on knowing your bin's specs. For a perfect, hassle-free fit, checking with your local municipal maintenance department or the bin's manufacturer is always the best first step. For generic bins, measuring and then searching online for bags with those dimensions will yield the best results.