I just saw one on Ann Taylor Loft's website!
i got a cute maternity raincoat from target.com a couple of months ago...
I got several good maternity jackets and coats on EBay. The other (and very easy) thing to do is get one of those long ponchos from the Totes people (in other words, better quality than just the vinyl square with hood available at the Sporting Goods department) and wear a normal warm outfit under it. Those full-length ponchos are great for football game in the rain even if you aren't pregnant. Try REI for higher-quality nylon ones, if you don't have any luck finding Totes ones.
Ah, and you are from Seattle...I am originially from Vancouver, so I understand the desperate need for a raincoat! I found a nice one at a maternity store here. It's called Thyme. Not sure if that chain is in the US or not. Now that fall is basically here, it should get easier. Try looking on line as well...ebay or on line clothing stores. Good luck!!