We scrounged a bunch of cinder blocks we found on the property and built my raised beds from those.
They are not very tall, of course, but considering how boggy my garden can get when it rains a lot, it raised the plants enough to keep them from rotting in the water.
My neighbor up the road couldn't set foot on his garden for two months last Spring and he had already planted some things. His garden is perfectly flat/level. Mine's on a slight grade so it's not as bad. One year when I didn't grow anything, it rained a LOT and I had a 1 inch hole in the garden area where water bubbled up for over a week. Clay subsoil, with rocks of all sizes and lots of Springs in the around here. Creeks and rivers were all high that year.