Here's the best idea ever...
Buy Wii Sports Resort, get one Wii Motion Plus packed in.
Then, Individual Wii Motion Plus units will have an MSRP of $9.99. Most retailers will sell them at $15.
There will be three different Wii Motion Plus packages. Each one will come with a download code (a la Wii Speak Channel) for a free Wii Ware download of a WiiPlay-esque mini-game that uses the Wii Motion Plus. You'll be able to tell which free game is "included" in your Wii Motion Plus package, as each one will have different packages. "Wii Motion Plus Fishing", "Wii Motion Plus Fencing", "Wii Motion Plus Something or another, I dunno....".
This will encourage retailers to keep three times as much stock on hand (who knows which version customers will want?) and will help to cause one particular version to sell out anyway and creating yet-another "must have" item, but without just flat out losing sales because you can't find it *anywhere* (a la WiiFit/Mario Kart).