When you go to your local nursery, you may be limited in what kinds of onions you can grow. On-line nurseries and plant catalogs are a good option to buy more exotic kinds on onions. However, always make sure that the onion is appropriate for growing in your area. This will help reduce problems with bulbing, bolting, diseases, etc.
Please see our posts on different onion types for lots of great ideas on onion varieties for your garden.