I use the Glad ovenware bowls that are 8"x8"x2" that come I think 2 or 3 to a pack and just use a knife or sisscors to cut a hole than take a nail file around the edges to smooth it out. They are black and my bp seems to love them.
I have also gone to my local carry out and gotten those black plastic bowls they use for carry out food which are about 6-7" round and about 2-3" high.
The carry out use to sell them to me for about 50cents.