Many Tribal belly dancers make their own tops by cutting up fishnet tights or shredding T-shirts.
That's fine if you're up on a stage, but if you're dancing up close and personal with your audience, you'll need something more neatly finished! A ready-made fishnet top will also last longer than the home-made version, which can start to look ratty after a few wearings.
Fortunately, dancewear suppliers like Discount Dance offer ready-made fishnet dance tops. They're also currently fashionable as clubwear, so you'll find them in the shops occasionally.
Fishnet tops on eBay