Those goldfish at the fairs are usually comets or shubunkins, which means they have the ability to grow over two feet when properly cared for. Yours is probably already not doing so great, due to the conditions it was kept in at the fair, and even before that, too.
One of those goldfish needs about 100g to themselves, although that's still not even enough. That's why these types of goldfish are commonly kept in ponds. The ones that can be kept in tanks are called fancy goldfish, and one of those fish requires a bare minimum of 20g, with double the filtration as goldfish are massive waste producers.
I highly recommend you sell it to a pet store or something, since even if you buy a fish tank, you'll need to cycle it, which takes up to a month, and as I said before, you'll need a large one.