I need suggestions for Flower garden borders

If you want plants as a border I would suggest liriope. Comes in green and varigated, white and green. If you want a SOLID border, gosh =, you can use anything, pavers, broken plates, stones....anything goes with that...use your imagination

What is your location ?What is the lighting?

You could use old soda bottles. If you wanted color, you could swirl acrylic paint inside them before placing them in the ground. Smooth river rocks make great borders too.

all the bordering and landscaping in my yard is done with rocks..Its natural, and of course free,lol, but if you are looking for a more clean cut lines you can look at your local lowes ,home depot or what home improvement store your area has and look at all the border stones, little fencing, I mean they have everything your looking for when it comes to this and the sales people will help you and even instruct you on how to use it ,..but yes it really allcomes down to your imagination..its endless

I like the look of big river rocks/boulders or brick.