Hi: I think where you buy your iPhone 3g they may have cases, or you can search on line to see what is available on or before the sale day.
If you're not sure what case to buy, iLounge.com is a great place to find out about new cases. Speck has nice cases as well as Griffin technology. but go to iLounge and read what they have to say about up and coming cases.
Your options are the AT&T store, the Apple store or on line. All of us are pretty much in the same boat mere days before the sale, we all want cases and have to wait. If the Apple store or AT&T have any they will be selling out I'm sure. And many of the on line sellers have not finished manufacturing their cases yet. We just have to wait.