There are lots of great ideas here: https://blog.gardenloversclub.com/gardens/10-brilliant-garden-edging-ideas-you-can-do-at-home/
Here are so more ideas:! https://www.landscapingnetwork.com/walkways/edging.html
Home Depot carries pavers in concrete grey, brown or red brick starting at 25 cents each.
Used brick is how I did mine. This coordinates with existing hardscape that was already in place.