Knee Brace support Hanger for Treehouse
I think those are Simpson Strong-Tie HD3B, or maybe a HD5B. At under 10 bucks a pop, you can probably afford to buy one just to look at it and see if you think it suits your needs.

edit: just looked at the fancy $100 ones, and I can see how you get way better attachment from them. I suggest you integrate a strap that ties the bracket to the underside of the brace, as a single lag through the bottom of the simpson isn't going to be stellar at resisting a force that pulls away.