scrap wood
2x4s are the most common boards. Stack 2 of em and you'll have a bed that's nearly 8" deep

fence boards
(Anna White has a detailed write up on her website) to make smaller beds/planters.

Scrap wood is wonderful, & you can find it pretty easily:

Cruise the alleys or curbs the night before trash is picked up.

ditto the night before the brush/'large item' truck picks up.

Check freecycle & craigslist;
if you don't see what you need, post a 'wanted' ad.

Make friends with a landscaper;
when a home- or business-owner changes the landscaping, sometimes the landscape timbers or edging has to go, & the landscaper can't use it on a job for which the client is paying 'new' prices.

Keep your eyes & ears open:
our local newspaper often advertises free pallets that you can pick up at their office.

Cinder blocks. They are cheap, never rot and you can rearrange or expand the bed if you want. The holes in the sides are great for growing smaller plants like carrots, basil and marigolds.

Used tractor tires. They are free, last forever, deep enough to grow any vegetable, easy to cut off a top circle with metal edge by a jig saw ( you need this cut for increasing the planting surface)

another

and another

and another

Concrete peaces work well and they are free. Look on craigslist or free cycle.it is expensive to get of busted up concrete so you would be doing them a favor. My neighbor has a raised bed made out of old concrete so I will take a pick next time I remeber

Also, you could reuse vertical 2 ft wide siding panels. They are easy to cut by scissors on 3-4 pieces and connect these pieces together as long as you wish. Then you place them into soil on desired deep/height and feel the bed with good soil mix.

Full view.

At first I used natural rocks that I found nearby, but I became a bit dissatisfied with the look. I found landscape blocks on sale at Walmart and Home Depot and used those. I like the look better.

CharlieBoring - How much did you end up paying for the landscaping blocks? I'm considering these as an alternative to wood since they last longer.

Thanks!