I'd think with a little work you could make something huge for much less than it would cost to buy something.
Since you'll be using Christmas lights, you just need to build your skeleton structure. A snowman is easy because it's just 2-3 balls. I'd get down to the home improvement store and see what you can get. If you are good with a jigsaw yo can get plywood and just cut 2 sets of circles out the largest diameter you can. Then add cross beams and secure them all to a pole. Then you'd have your first ball. Wrap it with as many Christmas lights as you'd like.
For things like eyes and a mouth, try to find a cheap glass ashtray or even large plastic jewel from the kids store. With all of the Christmas lights wrapped on your ball, it should glow like crazy. So, you just need to great a translucent color change for the features.
Anyway, I say make it yourself if you can. You'll have something unique. It will be exactly what you want or at least close. It will be cheaper than buying something that will do. On top of that, you can tell everyone, "Yeah, I made that."