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!