Music libraries are in the business of owning large amounts of music and "renting" (aka "licensing") the use of that music in projects. Most music libraries have vast catalogs of music in about every size, shape, genre, and tempo. In the last 5 years, music libraries have made some big leaps forward in terms of production quality, coming a long way from the old reputation of "canned music" they used to endure.
Today, music libraries are popping up on the Internet almost weekly. Separating the higher quality libraries from the lesser quality ones takes time and work. One quick way to take a snapshot of who's out there today is to Google the term "production music library" and see who comes up.