Designing is half the fun of making your own buttons! But unless you're a talented illustrator, sometimes finding right graphics to express your idea is a challenge. It's tempting to just do a Google image search and use something you find from there. But if you're selling your buttons, you could find yourself in hot water if the original artist ever finds out. Also, it's really just not cool to take other people's hard work and sell it.
A copyright is simply the right to copy. Copyrights are granted to the original author of the work. Just because an image is posted to the internet does not mean you can legally copy it. Yes, there are some fair use exceptions, and you can modify the art enough to call it a new work. But I'm not a lawyer and can't tell you if changing the color and throwing a photoshop filter on something you got off Google images is going to qualify for fair use. It's best to just play it safe and get permission. This goes for fonts too! Here I have compiled some resources for free stock images, graphics and fonts! Be careful to read the licensing for the individual art though, since some sites have a mix of both licensed and free to use graphics.
Creative Commons is an international network devoted to educational access and expanding the range of creative works available for others to build upon legally and to share. You can search for art based on the type of license which include Public Domain, Credit to the Author, etc. You can also search for works available for commercial use.
Pexels.com has an impressive selection of very high quality photographs that are completely free to use and modify without atribution to the original artist! Most of the photos have links to the photographers instagram and they ask for shout outs, which you should totally do, and do often, because this is a fantastic resource we should all heartily appreciate!
Pixabay is a bit better for finding graphics than Pexels. But similarly you can copy, modify, distribute, and use the images there, even for commercial purposes, all without asking for permission or giving credits to the artist. They also display "sponsored" content at the top, which are basically ads that take you to iStock photo for licensed images. But if you scroll down the page a bit, you'll find a great selection of art that you can use in your project without any worry!
Dafont.com is a great resource for finding free fonts for personal use. There are some available for commercial use, some require credit to the author, and almost all of them have the ability to donate to the author as well.