If you do a lot of arranging, you may be interested to know about the International Music Score Library Music Portal (IMSLP). IMSLP contains PDF files of scores that are in the public domain. Many composers of a number of genres are represented including a number of 20th Century compositions.
All files on IMSLP are now reviewed for their copyright status, but keep in mind that their status is labeled according to Canadian law (since the site is physically hosted in Canada) which states that pieces published more than 50 years ago whose composer has been dead for more than 50 years are in the public domain.
To participate in the project, you can submit a score, discuss a piece, correct a page, or translate into another language. Some of these contributions require you to be a member to prevent vandalism.
I have found this site extremely useful in my arranging work and also in my teaching.
