I recently got this one
but I'm not completely satisfied because of the high cost. He also sells lots of different models.
Here's another that I saw at NY Barbell
. I don't know anyone who has it. It doesn't look as long as the one I have but it's hard to tell. Mine is 42". I'm not good enough to train that wide yet. I can only do one pullup that wide so I tend to go with slightly wider with shoulder width for now. I hope to use a wider grip as my strength improves. I can't even do one wide grip behind the neck pull up as you see the guy doing in the photo. Maybe someday.
I think the important thing is that you get one that it is wide enough and that your elbows have space to go outward. I used to have one that just occupies the space between the doorway like this
. The problem with these is that the doorway interferes with your arms if you're trying to do use a wide grip with palms facing away from you. They work fine for palms facing toward you and shoulder-width grip.
I've heard that wide grip pull-ups are a great (if not the best) exercise for the lats as well as other areas of the back. Good luck!