I just picked up what has to be the best/cheapest mold I think you are likely to find. I mentioned that I had seen it (but didn't buy it for some stupid reason) on another post. Fortunately they still had it tonight.

Go to Target and look for the winter toys, sleds, etc.

They have a "Snowman Maker" that is a sturdy, two-part, red plastic 12" sphere mold. Right now they are on clearance for $4.98

It is made by Emesco Group.

My plan is to use a styro ball in the center to reduce weight.

I doubt they'll be around too much longer, so check it out.