If you want certain models, you're going to have to wait. The Apple Store at Chandler Fashion Center only had the 64GB Wi-Fi model while the Biltmore Apple Store had 16GB and 64GB Wi-Fi models. Since I only wanted the 16GB model, I walked away a happy customer. But, if you want a 32GB Wi-Fi or any of the 3G models, you're pretty much out of luck.
If in doubt, though, call ahead if you can. If you make a spur-of-the-moment decision and your store is out, see if the Apple Store you're at can call another location and check on their stock. Of course, this method only works when you live in an area that's lucky enough to have more than one Apple Store. Independent resellers or Best Buy stores are also worth a try.
The Apple-branded case is harder to find than the iPad itself. This is what I discovered at both stores, and it seems to hold true online as well. Glancing at the case offerings, I can see why. None of them looked very appealing, and only one other case had the ability to function as a stand.
My recommendation? If you want a good, basic iPad case without a stand, try the Cache from Tom Bihn. It'll set you back US$30, but it's a fantastic product and -- the best part -- it's made right here in the U.S from a company that's known for extremely high quality. If you want a case that can also function as a stand, check out our review of the DODOcase. It's a little more expensive than the Apple-branded case, but worth the extra bucks.
If you really do want to hold out for the Apple-branded case, the store genius I spoke with at Biltmore said not to bother going through the Apple Store at this time -- just order it online. The stores are selling out as soon as they get the cases in stock.
You can't pay cash for the iPad. I watched as two Japanese tourists tried to pay for an iPad in cash at the Biltmore store and they were declined. Like the iPhone, you need a credit card to pay for the iPad. Accessories can be purchased with cash.
Be sure to have a back-up credit card or debit card with you. The store genius processing my purchase told me that if your debit card gets declined due to your bank's fraud detection, they are not allowed to accept the same card -- even if you call the bank from the store and they approve the purchase over the phone. Thankfully, this was not the case for me.