Interesting question actually since its quite unusual for people to use fisheyes for such focal lengths. You could get a generic one that is screwed into the filter. If you look on the inside of the lens cap, it should tell you the filter diameter.
Most fisheye images are generally aimed at wider focal lengths so around 16-20mm so im not sure how the final image would look like when using a 50mm lens. The other consideration is a fisheye lens is aimed at distorting the image itself giving it a bulged look. What this in effect does is lower the image quality especially around the edges which is another reason why most images at 16-20mm look quite sharp because there is usually a lot of an image to squeeze in. The moment you start using other lenses, you may notice more distorsion and less sharper/appealing images.
I have yet to find a nice fisheye adaptor that I have been happy with but I can be a little too picky for my own good at times. Do not take it as a knock from me but more a heads up. You might come across a good bargain one online. If so please post some images, I would be quite keen to see the results.
