A good inexpensive way to get ambient music is through online royalty-free music sales sites.
Freeaudiolibrary is the owner of the exploitation rights of all musical works sold on this site. None of the musical works or their authors have any links with intellectual property rights management societies (SGAE, BMI, ASCAP, etc.). Thus, under the terms of a license included in the initial price, freeaudiolibrary grants without exclusivity the rights of synchronization and exploitation (reproduction, distribution, public communication and transformation) for all the uses specified in the license.
Royalty free music is an economical solution that allows you to legally add music with exceptional sound for your project.
If the music has been registered in a management entity (SGAE, BMI, ASCAP ...) then it is not free of rights and will have to pay the copyright. Most companies that claim to sell music free of royalties are not really, many times the payment of the purchase will have to pay the royalties.