For SNES Classic, Gamestop has been getting them in. They were showing up yesterday in stores, but I think they have been continually getting them. Talk to the sales guy... ask about the system. If he's cool like the person I dealt with he'll call around the other stores that show they have them and will get one transferred to your local store.
Last Friday Amazon had a preorder up for the SNES Classic as well as Gamestop. Keep an eye out ... they could do that again.
Beyond that, use your phone and pull up the Gamestop website, look for the system and look to see which stores say they have a new one in. For some reason the desktop version of the site doesn't show stock... but the phone version always seems to work for me. Also brickseek for the system with Walmart and Target. There was Walmart about an hour away that had two, but it wasn't worth it to drive or call and hope I got someone that would hold it (let alone take the time to verify they had it). Also when brickseeking, there are two sku's for the SNES Classic with Walmart. Google should reveal both links. The Gamestop option overall was the least painful, especially since it's super close to work and when they call I can just go pick mine up. Preordering would have been easier but by time I decided I wanted one for sure they were sold out and GS had an estimated delivery date when I'd be out of town. Given it's rained here pretty much every afternoon, I didn't want to risk a package showing up at that time.