The float bowl of the carberator on my ATV fell off while I was riding?

Your local hardware or motorbike shop should be able to supply you screws that will fit. Bring your carbie if you can so you can try them for size. You don't need genuine parts. If it's a made in China ATV it will probably have metric fasteners. Screws that size are cheap. See if you can get a few spares. For ones and twos they generally aren't worth doing the paperwork. You may get given a handful for free.