I got a very sturdy one from our local pet store but it wasn't cheap. It was about the same size Spikey mentions, and it holds a full grown 21 inch Aussie Shepherd with enough room to lay down and sleep comfortably stretched out.
We used it as a "den" for our dogs when they were puppies and now we use it when one of the girls is in season......the boy gets to sleep in it. other than that we don't really use crates at home, although I use a trolley and a secura pen at dog shows.
My suggestion would be to get one with a metal tray in the base, not plastic (or you won't have a base for long) and buy a size for an adult, not a puppy.
Ebay is fine, or if you're near somewhere that regularly holds dog shows, the vendors there have them and they are often surprisingly reasonably priced. And you can try your dog in them for size.