I have occasionally seen these on Ebay but you have to keep checking.

You can save some money by buying metal brackets at HD or any place that sells lumber. They have what you need in the lumber section rather than the hardware section. They are galvanized angle brackets that have a bunch of holes punched in them and are used to join framing lumber like 2x4, 2x6, etc. You need brackets about 5" high for use with 2x6 lumber and they are about $2 each if I remember right.

Sorry, you can't just slip the lumber in them. You use the brackets on the inside of the corner and screw into the lumber with galvanized deck screws (about 1-1/2 inch long). It is really fast with an electric drill and a philips bit; if your lumber is really tough you might have to drill a pilot hole first but it is usually not necessary. You can disassemble them easily but it is not something you want to do frequently. They don't look great but are usually covered with dirt anyway.

Bob B

Lee Valley sells a set of 4 for $9.50 + shipping. $8.50 if you buy 5 sets or more. These are actually not designed for this purpose, but are really designed as "pallet hinges" aka "pallet bin hinges". http://www.palletfactory.com/index.html has more information. But I have been unsuccessful finding a US source that sells in small quantities.

Bobb has a great idea about using galvanized framing brackets. Take a look at the Simpson Strong Tie catelog for inspiration. I use joist hangers (face hangers) at the corners and a few RC style clips to hold courses together.

VT

Here is a link that might be useful: Simpson Strong Tie

The price of a set of 4 hinged corners from Lee Valley is now $12.80 ea set, or $11.50 for 5 or more sets. Still considerably less than brackets I've found elsewhere, and an extremely useful product.

Here is a link that might be useful: Lee Valley Tools

Magret,

My raised beds are not fine furniture. I made them from former 2x6 white oak boards that were part of the tobacco racks on our farm. My beds are 6", 12" or 18" high and 10-25' long. Boards are NOT attached at the corners, as my soil has much clay, and I really want drainage at all times in my area. I use steel rods, recycled steel posts and even pieces of metal conduit as stakes to hold the sides in place. Some of the boards have slight curves from age. For me, these are the best beds I have ever had. I will never fasten corners again. Might work for you.

I will be exchanging cinder block for my wood beds beginning this year. My friend uses 8 inch blocks to border his raised beds and never has to touch them. In addition, he fills the holes in the blocks with soil and grows herbs/flowers in those.
He found his blocks(100 of them) in a shopper's paper for $1.00 ea. Says he found others for 50 cents, but they were used. What do I care if they were used?