Hi Moon,
Have you tried looking at the "Frugal Gardening" forum? There is a current topic on that [it's on the top]. It may be of some help. People have suggested anything from kiddie swimming pools to bricks, concrete, old tractor tires.
Take a look!
Good luck, Cat

Have you tried a google search?

If you will go down several posts to the "raised bed alternative" you can see what we use.

Scavenged brick. Boards pulled from loading pallets - most bog box stores will give away damaged pallets. Scavened boards. Old bookcases (some, uncomfortable with the laminate or wood treatment, line the bookcases with plastic). Old bathtubs. Old dresser drawers (line as with bookcases). "Airwalls" aka raised beds without walls at all, just heaped up sides. Scavenged stone. Discarded bits from the nearest home building site.

You can have any two of cheap, easy, quick; but having a solution that addresses all three is tough.

Harimad

I have two 4x4 beds that I made with Trex, a gray wood replacement that sounds like it's similar to your TerraBoard. I got mine at Home Depot, where it's sold usually to replace wood in decking. It's more expensive than wood but will last just about forever, and it looks nice, a consideration since the beds in question are in my front yard. For the back yard, I'll use anything I can lay my hands on - I started using bookcases and kiddie pools years ago.

If cost is your most important consideration, the absolute cheapest option is to not use walls at all, just slope the sides of the bed. Next cheapest is scrounged stuff. Trex and TerraBoard are not cheap but will last for years - if you're in a position to make permanent beds and appearance is a consideration, even the high cost of the wood substitutes might be acceptable when spread over several years.

The bender board/TerraBoard actually is really cheap - $10 for 40'. But it's a flexible plastic edging strip, so it does need support, and is more of a temporary fix.

This was cheap and easy for my strawberry bed (6'x6' for convenience of building this bed). I bought 4 6"x6' redwood fencing pieces @ 1.79 each, and 4 12" surveyor's stakes @.13 each. Used the stakes as corners, positioning them so they closed up the little gap from the notches at teh top of the fencing boards. So the 6x6' bed set me back less than $8. So far it looks great, and went together in about 20 minutes. The stakes hold it firmly in place. These beds could easily be used for 3'x6' beds with no waste. One bed would be less than $6.