As far as fence posts are concerned. Lowe’s, Home Depot, 84 Lumber, and any other lumber distributor will have treated 4x4 posts up to 16’ and 4x6 or 6x6 for that matter. All can be ordered and delivered if you don’t have a way to haul something that long. Also, look into custom sawyers in you area, they may be able to cut something out of a rot resistant wood like larch or cedar, cut to your specific needs.
As far as attaching the welded wire to posts, I use fence staples. Available at any farm supply store. They are usually hot galvanized and will outlast the fence material.
Under my chicken coop gate, I attached a piece of mud flap from a truck, that is about 1 inch longer than the opening under the gate. The mud flap is stapled onto the back of the gate, and when the gate is moved, the mud flap bends and moves with the gate, closing off the opening. If I can keep weasels out of a hen house using it, you should be able to keep woodchucks, ground hogs, whistle pigs, or what ever you call them, out of your garden. Also, a .22 works well for that application.
I will say that burying fence under the ground about a foot does work well to keep burrowing critters like groundhogs and rabbits out of the enclosure.