I know you said plastic/resin, but any of the companies that do lasercut mdf or plywood could get you what you need. Just make sure you use 3mm thick (the hight of a slotta) material which most companies offer.
Of course they won't have the chisel edge of a slotta...
Unfortunately, the Magnetic Displays item in the link says "round". I don't think that listing is for a square base. I think it's for the round base in the right of the picture. The square base in the middle looks like a standard 20mm square base. I don't think they offer square 30mm bases.