Check out this article: https://www.fotor.com/blog/11-best-stock-photo-websites-to-improve-your-visual-content/

In this article you will find stock photo websites that you can use as backgrounds for your Instagram Templates if you don’t have any of your own just yet.

Design Tip: Choose stock photos that relate to your brand and put related quotes over top of the image.

Here’s a link on a tutorial on to create a Instagram template for your page:https://iblog.iup.edu/cpsw/tutorials/