We have an Agri Supply locally that I buy all that kind of stuff from. I built a set one time but they mass produce them and by the time you count your time, materials, wear on drill bits....some things are cheaper to buy and I find these arms fit in that catagory.
Most any farm equipment dealer will sell parts to fix it but the bigger chain dealers should be cheaper.
You also can do like I do on a lot of my small equipment. I have a trailer mover hitch, (reciever tube hitch) for my tractor. It has that triangle already made. Cut you a piece of 2"X2" square tubing to correct length and weld it on to the seeder, and then work it out of your reciever hitch. When you're done, pull one pin, slide it out and use the hitch for something else.