How tall are your beds? I've built raised beds from a wide variety of materials and in a wide variety of sizes. My oldest beds were the most ambitious design. They're 18" wide 2 inch thick cedar panels, glued up from lumber cut from trees that were destroyed by an ice storm. There's a 5 x 5 "post" at each corner and the beds are 4' x 4'. They're gorgeous, but they were a ton of work. The effort was monumental but the design used the downed trees efficiently. A few years later a wind storm (some say EF 0 tornado) took out several more cedars. This time I had them cut into 4 x 4's and laid the beds log cabin style. These beds aren't as efficient a use of the lumber, but were much easier to build and will last a long, long time. You could do the same with landscape timbers, which quite often go on a pretty serious sale at the home centers.
The last beds I built, I just took some big cedars that the neighbor knocked down with his dozer, cut them to length with my chain saw and buried them slightly. They're a little over 10" deep and look very attractive in the garden.
If you've got trees, use them. If you have to buy...there's no cheap road.