Shortly after the Sidekick 4G was released moonrock noticed that the part number on the SK battery was the same as the part number listed online for the Samsung Intercept's battery.
I took a chance and ordered one from a seller on eBay, and it works! It's currently running my phone.
Two notes: I don't recommend the eBay seller I used. They raised their prices five-fold when I linked them here. I've deleted that link. Just search for a Samsung Incept battery.
The battery I received is marked as 1500mah, but it weighs 15% less than the stock battery (27g vs 31gr) so I can't imagine it actually has the same capacity. Most aftermarket batteries are crappier than the OEM one, but at least they're cheap crap! Buyer beware.
(I originally asked this as a question in this forum. Now that I know the answer I'm now posting this new thread to get this tasty information out there. Incept batteries are cheap!)