You can get them for £20-£30 or so off ebay.
I have one and it's a fun enough thing but don't expect the highest quality. I can't justify the cost of a fisheye to myself as I know I'd hardly ever use it but a cheapo adapter is fine to play with and will give you a glimpse of the fish eye experience.
Oh, and no offence...but I don't think that these adapters are anywhere near as bad as some people say. If you think they're a cheapo alternative to a dedicated fish eye lens costing hundreds of pounds you'll be disappointed but view them as £30 well spent on a toy and photographic curio for occasional fun use and I think you'll be happy.