One method I haven't seen mentioned is DIY lamination. Rip your 2x into 1/8" to 1/4" thick strips, depending on the sharpest bend. Cut a jig to the shape of the inside surface of the rib. Start gluing the strips together as you clamp them to the jig. Once the glue is dry, they will be the shape of the jig. I like using the 1/4" narrow crown stapler as a clamping tool. After you pry it off of the jig, snip off the legs that show through (use SHORT staples).
Its slower, fiddly, but you get very strong ribs for low cost. If want, you can even make the outer layers wider so you have, in effect, and I-beam.
Good quality plywood is the fastest, and seems to be the most common choice.
You'er probably done already, but I thought I'd offer another option...
Michael