Hi!
I have the same problem with my low-to-the-ground Artie -- he's a Lab mixed with either Corgi or Beagle. Either way, his legs are short and he always comes home with a wet belly when it's raining (or has rained and the wet splashes up).
Regardless, he'd NEVER let me put a jacket on his. I'd love to, but... them's the rules.
I Googled
dog jacket belly protector and came up with this, that looks like it could work for you:
Zack & Zoey Ripstop Ranch Dog Coat | Overstock.com
Oh, and when you click the link, look at the photos on the side, too. I think those are different jackets, but there's a lot of coverage on those. They all seem to be called "ranch" style coats