One strategy you can use is to find public domain content for your project. Anyone can legally use these works in their projects without permission. Works may be in the public domain because intellectual property rights applying to them may have expired, they may have been waived, or the work may have been created before copyright law existed.
A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
Keep in mind that, even if a work is in the public domain, it's best practice to provide attribution in your project's credits.
