I have used sheets, both new and from the thrift stores. I have used full width fancy backing. I have pieced backing in various ways.
Just be warned that very few things show every flaw as well as a white sheet. Still that motivated me to make my sandwich (backing, batting, top) as perfect as I could.
I do recommend prints!
Not sure how long this sale is going on, but eQuilter, my usual go-to store, has the wide backs on sale now at 25% off. Be aware that dealing with eQuilter you get exactly what you order, so not an inch extra. Other than that, I've been dealing with them pretty much since they started 20 years ago and I keep going back.
https://www.equilter.com/category/265/quilt-backing