Johnyseeds has a good selection of these.
I have one that makes 3/4in cubes to start seeds. I have a 2 x 5 heat mat, with a plastic sheet and a drawn grid on top and can start somewhere around 1,000 seedlings at a time in that 2 x 5 space.
Plans are nothing; planning is everything.
We don't see things like they are, we see them as we are.