The quickest way I found out to check local inventory is do the custom style picker and choose a bunch of different case and band combinations you like. Then click on each one, to see availability. In the "Order today, delivers to" shipping area if you see "Today" or "Tomorrow" then there is stock in your area, then click through to see which store below in the pickup section.
One thing I noticed is that the Apple's preset combinations on their main page are actually shipped pre-packaged (like Starlight case and Starlight band) and may be harder to find when case or band sizes aren't available. Other custom combinations let the store put together the kit themselves using available cases and bands in the store. I saw this when I picked up my Midnight case with Midnight leather band. The back sticker on the package only said Midnight Aluminum Case but didn't mention the strap. Whereas my wife's Starlight + Starlight case had both the case/strap combo on the sticker. I had also picked up Midnight Case + Abyss Braided Loop combo from another store and it was the same thing. When I opened the package, the Abyss loop inside box had a price sticker on it which meant they picked up from local stock. So if you do find one case/strap combo they have in stock they may have other combos with the case or the strap as well.
The hardest to find are the stainless steel silver 45mm, I think these are the most heavily backordered. Aluminum combos seem to be easier to find since they get more inventory just have to be diligent to daily.
