Forklifts can put a lot of pressure on their tires. Partly this is because they're lifting so much weight - a forklift tire has to provide a lot of support, and over time there's a good chance you're going to lose one or two. Another factor is that they often are being used in areas with lots of little hazards around. Construction sites, warehouses, industrial areas, etc. are the usual locations - but they also usually have lots of stuff lying around that could puncture a tire. It's often a good idea to keep a few replacement forklift tires on hand just in case, because if you don't and you get a flat or a puncture, you're going to be out of commission until they ship.
You can get replacement forklift tires in a couple of places. First, your dealer will usually sell them. Second, they're easily available for sale online:
Finally, you can look to online dealers in forklift parts. There are some good dealers listed on that page, but you usually need to know a part number.