My batteries are only three and a half years old. Most UPS batteries are rated at either five years or ten years. It isn't a good idea to keep spare batteries on hand unless you have a way to keep them on charge.
Most VRLA batteries are readily available.
You should NOT mix different types, brands or models of batteries because the charging and discharging characteristics will most likely be different. This usually results in an incorrect float voltage on the replacement battery because the impedances are usually different between the batteries. This will lead to an early failure of the replacement battery. My battery should have lasted more than forty-five minutes, but during the last power outage they lasted only six minutes. We had all of our batteries tested and they load tested good.