Bestbuy?
I spoke with a Bestbuy representative about two days ago, and they said that they'll be selling the iPhone 3G on September 7th. Therefore, they're expecting some iPhone accessories within the next few days. With that said, they didn't mention if they'll specifically have the Incase slider for the 3G iPhone available, but they do have the rest of the Incase accessories on hand, so I don't see why they wouldn't have the slider case available.
If all else fails, you can always try eBay. Their shipping and handling is only $5 compared to the $10 Incase is charging.