Adventure RV has Blue Ox base plates for about $380 for most models. They also offer free shipping sometimes.
For auxiliary braking, I recommend the simple, low cost,
ReadyBrake System. If you haven't bought a tow bar yet, the
ReadyBrute Elite is an excellent 8,000 lb rated all-terrain aluminum tow bar that has the ReadyBrake system built into it. That combination plusa Blue Ox base plate has worked well for us for over 40,000 miles towing two different toads.