The best choice is the Empathy Belly. This is a device you can get to simulate a pregnancy. You get it from the hospital, put it on your husband and let it kick him around for a few days; then he'll "know" what pregnancy is like for you and will supposedly be nicer to you. (It's only a partial simulation--it doesn't come with hormones--but it's supposedly pretty good.) These are the best choice but they're nearly $500--if you can't borrow one from the hospital for your production, try...
Some theatrical stores sell pregnancy prosthetics. They look really good.
Halloween costume stores sell padded bellies; they look okay.
Or if you want to get cheap, just go to a fabric store, buy a pillow form for a round pillow and go forth from there.
The other option is to cast an actual pregnant woman in the role.