A lot cheaper to build your own that way you no what you have and if it will be strong enough, ect....
I had the guys at the shop to weld me up a hinge plate, mine is made out of 2 plates, 1/2" thick plates and a foot square, had a 3ft piece of 3" pipe welded on the 1/2" thick plate, and drilled a hole through this pipe about 2ft up from the bottom....this didn't cost anything got this all from scrap..
Now will go with a 21ft joint of 2" pipe, this will slide inside the 3" thats welded on the plate, and will drill a hole in this 2' pipe about 3ft from the top, and this pipe will cost me 102.00 bucks for a 21ft joint of 2" galvanized.....
Next I will go with a 1.25" galvanized 10ft piece of pipe that will slide inside the 2" pipe, this sells for 3.49 a ft here...already had this piece of pipe..
Will drill a hole in it and use a grade 8 bolt and run it through both pieces of pipe and just going up about 27ft, thats all I can do....going to go with a 3 element beam on mine, and also set it up so I can run a dipole or 2 off it...
Total I will have in this around 160.00 bucks...
And the hinge plate the guys welded up, they say it will handle all the weight I wanted to put on it.......
Mine is over kill, the guys used a brace 1/4" thick on all 4 corners of mine, and the bottom plate also has a 3ft piece of 2" pipe welded on it, this will be in the ground 3ft with cement around it...and I used a piece of drill steel to make the hinge bolt out of, this is some tough steel won't break...
But I would suggest having you one welded up and that way you no what you have and how much weight it will handle....had this onemade to be heavyduty, due to the situation I'm in here...