Not sure how heavy you're looking at for capacity, but when I got rid of my tractor I still had a 3PH boom pole for it. Had a shop weld a SSQA plate to it and now use it all the time on my skid steer.
Here is a plate for you to start making one.. or like the previous poster, buy a 3pt boom pole from TSC or other farm store and weld a plate on it. For what they want for a ready made one. you could buy a 3pt boom pole, the plate AND the welder, and still be money ahead. And have a welder when you were done.