Howard Precision currently has cast and wrought plate up to 12" wide 60" long on sale for $1/pound (plus shipping if you don’t live nearby); minimum order $30. This includes Mic 6 and Alca 5 cast plate, both of which are good choices for constructing heated beds. The drops I have purchased from them before have arrived in better condition (impeccable) than custom cuts from Midwest Steel (dented and scratched) and I highly recommend Howard Precision.
Note that you have to request a quote, and sometimes someone else got to a drop ahead of you. My experience is that they’ll try to find something similar that meets your needs. Then you have to fax or phone a credit card. (They mostly serve businesses, but have been very helpful and friendly to me as a hobbyist; they just aren’t set up for internet click sales.)
No affiliation, just a satisfied customer.