That's a great question about maintaining public bins! Yes, in most cases, the plastic liners for those standard city trash cans can indeed be purchased separately. They're not a permanent part of the bin. You can typically find them through the original supplier of the trash receptacles, at janitorial supply stores, or even online through retailers that specialize in commercial-grade products.
As for sizing, many public trash cans, especially those from major manufacturers, do use standard-sized liners. Common diameters you'll encounter are around 18, 21, or 24 inches. However, it's not a universal rule. Some custom or designer bins might require proprietary liners. The safest approach is to check for a manufacturer's label on your specific bin, which often lists the model and compatible liner size. If that's not available, simply measure the interior diameter at the top of the can to ensure a proper fit for the bags you plan to buy. This makes replacement and maintenance much more straightforward.