This is an important first step in arranging works for your group, and permission must be obtained from the publisher who controls the copyright of the piece. Most of my pieces are published through Tapspace. You can get information on their process here.
If you want to arrange a piece of music for your indoor drumline (for example), you'll first need to find out who it's published by. One of the best places to start is to search the ASCAP or BMI websites. Often, you'll be able to find the publisher information (and contact information) there.
For more information on this, and other types of related copyright questions, the NAfME website has some great copyright resources for music educators which spells things out in pretty simple terms. Also, the Music Publishers Association has a good page of resources here along with a stock set of common forms that you can use to get started once you've identified which publisher to seek permission from.
In most cases, a permission to arrange license is limited to the group who obtained it. So you can't necessarily just re-purchase the scores for something someone legally arranged for a different group in the past. Even if you were going to use their arrangement verbatim, you should still acquire proper permission to be in compliance. For example, if you wanted to adapt the 1999 SCV arrangement I made for "The Canyon" by Phillip Glass, you'll need to first obtain permission to arrange from the publisher before going down that road.
If you'd rather not deal with it, Copycat Music Licensing has been serving schools and organizations such as DCI, BOA, WGI, and many more secure their music licensing needs. They can help take away the legwork for you.
Lastly, an article about legal stuff wouldn't be complete without a disclaimer. I am not a lawyer so don't take my word for it. The information here is not to be considered legal advice.