The base plate may take a while, although it is a common toad, but try haunting craigslist for it, the tow bar and braking system. You can set up alerts for key words like Blue Ox, Roadmaster, tow bar (and towbar), etc. I found a nice Blue Ox tow bar that had only been used four times for less than half the new price. Most are pretty well used but you should come up with something in a couple of weeks, just don't wait to respond to the ad.
I'd be a little wary of purchasing a used electronic braking system from someone I didn't know. I ended up buying a new, all mechanical ReadyBrake system and a new baseplate.