I was thinking I was going to have to DIY this year, but I found a giant whoopie cushion costume at Walmart for $15! (It was marked down for some reason...it lists it for double the price online.) They have a lot of clearanced costumes that would fit a pregnant belly.
https://www.walmart.com/ip/Woopie-Cushion-Adult-Costume/14698812
Re: Found an inexpensive halloween costume...that fits my pregnant belly!