I have the same problem with mine, the company that made it stopped manufacturing them about 30+ years ago. Its kind of funny, because they say they are available, they just don't have anyone left that remembers which foundry has them.

You may have to modify something that is close to yours to fit your unit, maybe just the shaker grate, or possibly the grates and frames together. It would be a bit pricey to have new ones cast to say the least.