It has begun: pre-orders for the SNES Classic Edition have now kicked off at GameStop, although in-stores only. However, Target is right now taking pre-orders on their site and the console is still in stock. You can find it at Target here.
So far the console has yet to appear for pre-order on Amazon or Best Buy in the US. We’ll update this page as soon (if) it goes live there.
Right now your best bet is to go down to your local GameStop and get a pre-order.
And be sure to check the new SNES Classic Edition trailer released earlier today.