I'm laying out a "four-square" garden at my place in central Maine. I'd
like to use some kind of precast concrete planks to create the
rectangular raised beds, but I haven't found much.

Anyone have any sources for concrete raised beds? I'd like to stick with
long planks if possible so I don't have to do as much fussing and
adjusting as they heave in the ground.

Thanks for any advice.