Does anyone know where to find a decent half sheet size cake platter?

"platter"??

Such as a ceramic, glass or metal platter?

Most here would use multiple cakeboards, cake drums or foamcore (or even cut & covered plywood) for a cake that large.

Also what dimensions do you consider a 1/2 sheet since customers often mean anything from a 7x11 to 12x18.....

HTH

A half sheet cake is 12x18 so I hope that is what you mean. So the platter would need to be about 16x22 or so. That would be one heavy, ginormous platter. You might consider using a rectangular serving tray, like you'd carry drinks on. My husband cuts 1/2" plywood cake boards for me for that size cake.

I use foamcore cut to size and covered for cakes such as that. Strong, and not heavy.

I use cardboard cake boards--three of them stacked and wrapped with foodsafe foil. (The first time I made a half sheet cake I learned the hard way not to miserly and use only one----my husband carried the cake on his lap during the delivery and flexed it, the buttercream rumpled up like elephant skin.)