We have a built-in, homemade fibreglass shower that we want to paint. Over the years its just gotten old looking and we want to refresh it what can we use?

Nothing will work. Some paints may say they will, but they will take more cleaning and sanding of the fiberglas than is worth it, and you'll never get the corners. Just rip it out and pop a new one in.

That doesn't help so much as it's a home-made built-in that we can't find anything to go in there as it's a very, very small shower.

Well in that case, sand well, clean with VM&P Naptha, use a good epoxy enamel. May last a few years.