If you want to sell printables, then the art you use has to be legal for this purpose (See more on this topic here). There are several ways to acquire art that you can use in items you want to sell; CreativeFabrica is my absolute favorite. It is the only site I literally use daily – and if I sound biased, well, I am 😊
Note: Some of the links below are affiliate links. This means that, at zero cost to you, I will earn an affiliate commission if you click through the link and finalize a purchase.
The sunflowers set below is just one of the thousands of pieces of artwork that can be used for printables -- I've actually downloaded and used this one already! It has a wonderful selection of images with transparent backgrounds and works on everything from junk journal pages to printable planners. Find this one here, and look around while you're there, CreativeFabrica is particularly flower heavy!
Is CreativeFabrica Right for your Printables Business?
If you’re just starting out, I always recommend starting with art you can get for free (and legally use). Your own art, public domain pieces and items that are out of copyright are most easily used and they’re free. When you’re starting any new business, you need to keep costs down. Since I want you to be profitable quickly, I always recommend starting with free sites like Wikimedia Commons (see the full details on this huge collection of free images here).