What can I use for borders around flower bed?

This is what I used around my garden where I have raised beds. We're in the country so we used what we had a lot of, which is trees.

https://www.gardenupgreen.com/2018/03/create-a-tree-trunk-border.html

Here's another peak:

https://www.gardenupgreen.com/2018/07/gathered-rocks-for-a-walk-path.html

We just use large rocks to create borders around our gardens. Small pavers work great, too!

We use garden blocks glued together with PL Premium for a nice sturdy wall that will last forever.

Using a square shovel, or edging spade, create a natural edge that is about 2 inches deep around the beds. When you mow, you can zip around the beds. Zero cost, unless you don't have a shovel or spade.