Where to get one? I'd try your LS dealer, if you value your time more than a few extra dollars, as they'll have one already configured for your machine. If you want to go the DIY rate, look for something around 400 watts with a plug diameter matching that of the most conveniently-accessible freeze plug on your block, and a cord of suitable length to dangle out from under the hood shroud.
These things usually take only 20 minutes, or thereabouts, to warm the engine sufficiently for an easier start on a cold day. Since my coolant is mixed for weather way below what we'll ever see, I only ever bother plugging it in 20 minutes before using the tractor on days below 10F. On snow days, this means plugging the tractor in first, and then shoveling the walks before firing up the tractor to do the driveway.