I love the look of using stones to border a flower bed. Farmhouse on Boone found hers for free at the creek bed on her property. https://www.farmhouseonboone.com/farmhouse-on-boone/lowes-curb-appeal-makeover-farmhouse-porch-reveal/
You can buy plastic edging like this https://www.lowes.com/pd/Suncast-Dig-In-60-Ft-Resin-Edging-60-ft-Black-Plastic-Landscape-Edging-Roll/1000573743 that's inexpensive and easy to install. They also sell it in smaller rolls for a little more per foot. The only problem with the plastic edging is that it can chip when using a weed eater.
Metal landscape edging cost more but lasts longer. https://www.lowes.com/pd/COL-MET-8-ft-Brown-Powder-Coat-Landscape-Edging-Section/3015399
You could use the paving stones which interlock and look nice or you can use the roll plastic. The stones would be easier to lay as you only need to level out the outside so they sit nicely.
Mine are used bricks so that it coordinates with existing hardscape on the property.
Hey Nora, check some ideas here: https://www.decorhomeideas.com/garden-bed-edgings-you-can-diy/