I have modified a couple of bikes to move to rigid forks. One that I use(and like) is the Tange. That fork along with others are available here:

http://webcyclery.com/home.php?cat=319

It looks like your bike may have a threaded headset... In my case, I had one that was threaded, and I went threadless at the same time, which took a full headset change, and a new stem.

It is something that can be done on your own, but a shop would probably do it cheaply, especially if you buy the fork from them, and they have the right tools.