You can get them at your local boat shop. However, I don't have a real local shop (no place on the way home), so I ebayed mine.
Marinco On-Board Charger Inlet 150BBI - eBay (item 370094352737 end time Feb-04-09 06:35:35 PST)
It is just wired to the block heater.
The block heater itself is just the OEM optional Toyota block heater.