Transposed parts; younger students love them, but on the whole they are a crutch. One of the reasons they are used so often is because students have either had only a limited number of lessons or not enough transposition practice in the lessons they have had. Another reason is the collection of all four Mozart concertos with transposed parts is so popular and is the introduction to these works for many young students. It takes some prodding to move on and use the real parts.
There are many published editions of these out there to choose from (I personally am partial to the Baumann edition of the Mozart horn concertos, published by McCoy’s Horn Library) but also a very workable, older edition in the public domain may be found as an online download in the IMSLP project. For example the horn part of the old Kling edition of Mozart 4 may be found here.
UPDATE: See also our Horn Matters PDF library.
Transposition is a very important skill every hornist needs to learn. I include exercises for transposition in Ultimate Horn Technique and there are many sources that work well. It is a technical skill that can’t be ignored. Whenever possible perform from the parts notated in the original key.
The only place where I would say to maybe think about a transposed part is when playing Wagner tuba on for example Bruckner 7. The standard version of the first Wagner tuba part for example is in B-flat alto and starts out in the key of six sharps with two double-sharp accidentals in the first bar. You have enough things to worry about with playing a Wagner tuba at all that a transposed part really is not a bad idea there. See this article for more on Wagner tuba.
You may think of other exceptions but again, speaking generally, always use the original parts. You will find them in the long run easier to read, and the skill will pay off in the world of actual horn playing.