Concrete

Using "old" concrete stacked like bricks to create any height desired is a popular form of landscape borders. You can get old concrete and just break it up into the size you need with a sledgehammer. Once you get it stacked to the height you want (no mortar necessary), you can plant draping plants at the edge so they cover the look of the concrete if you prefer, or leave it bare.

Go to your local waste dump to find old concrete, with the landfills trying to cut back on how much garbage is dumped everyday, they will charge you very little, if anything for the concrete. Another option is to keep your eyes peeled for anyone re-doing their driveway. If you ask, they'll likely gladly give you the concrete they are pulling up.

Go Rock Hunting

If you do not want to spend anything on your landscape borders, but you do not mind spending a little time, go rock hunting. Rocks make a very natural, and lovely border, and cost nothing. Look for your rocks in areas where there is a lot of construction. Construction usually involves the builder digging up the land and then raking it flat, which brings up a lot of nice sized rocks. Most builders are happy to have you remove these rocks, as they don't have to haul them away. This will give you a variety of rock sizes, and if you don't find enough you can always go hunting for rocks in the mountains etc.

Variety

Landscaping borders can be made up of more than one material, and are still going to look awesome. So, take a walk in the woods or along country roads and you'll find all sorts of rocks, pieces of concrete, interesting logs, etc. that can be used for a landscaping border. Add is some large shells or other items you may already have, and this will make for an interesting edging, especially if you have some stories you can share about how the edging came about, where you collected the materials for it, etc.

Visit a Quarry

Using scraps of rocks and other materials is a great way to make an attractive border, so try a local quarry and see what they have to offer. Often times the small leftover pieces are placed in a scrap pile and you can take as much as you want (and can haul). You could have a granite border, and get it for free.

Recycle Some Tires

Try using tires as your borders, this may not look as nice as you want, but it doesn't always look bad, plus they are indestructible. They don't rot, dogs don't chew on them, and you can get them free almost anywhere. Simply cut the sidewalls off of a used tire, and then cut across the tread. This gives you a two to three foot piece of edging that can define your border. And, depending on the tire size, they can be up to 6" high. You just need to be careful of the wires if you cut a steel belted tire because they are sharp!

Use Railroad Ties

One option for borders is railroad ties. You can usually get these for free as the local railroad office usually gives away the discarded crossties as they replace them. They are very heavy to move, but generally if you haul them off they do not charge you a thing.

Get Creative

You do not have to have expensive, or even typical borders, get creative with what you have, use old rain gutters turned upside down, and bury the edges. You'd be surprised how well this works.

Cut Logs

Another great option for inexpensive landscape borders is cut logs. You can usually obtain these for free from arborists, landscape companies, or by looking around in your neighborhood. Select large or smaller logs, depending on the length or depth of the bed. This will give a very natural looking border.