Not sure where to buy, but something to watch out for...
Distant Uncle has a 41' Columbia in Puerto Rico, few years back he bought a set of davits, they may have been custom made at local shop, im not sure. He has 9'rib and a 15hp yahama 2 stroke.
Around 10:30pm on our first night out of a 2 week trip we get caught in a little Squal, blowing about 35kts. Waves pick up, and the stern davit breaks. The hole where the shackle for the block attached was drilled too close to the edge of the metal, and it ripped through.
Long story short, to prevent the dinghy engine from damaging the rudder (it was banging into it pretty hard with every wave, and was hard to hold it straight), and leaving us really screwed (the alternator had died earlier that evening too), i basically had to jump overboard to release the lines from the dinghy hard points and extract the dinghy from the rudder. This was at night, in a storm, with driving rain, while going through a cut in a reef a couple miles offshore....
Make sure whatever davit you get wont break like this!