I buy all my wigs online or sometimes in person in a merchant hall at an event. When online, I usually buy from eBay, Arda Wigs, or Wigs-us. Another place a lot of people are getting wigs is Fantasy Sheep and C.C. Kids from Taobao but you would need an agent and perhaps a group order to make it really worth ordering that way. Hope that helps a little.
In step 3, I'm guessing you cut the bottom layer into the shape you wanted with a perpendicular cut (like how you'd normally hold scissors), but just in a V-shape, and then you trimmed the hair into a more natural look with a vertical cut. But what about the upper layers? Did you just trim like crazy until all the hair got trimmed into a V-shape?
You guessed correctly. For the upper layers I continued vertical cutting using thinning shears as well as regular scissors. This is a very basic layering technique.
I have since discovered that using hair trimmers with different combs can get the layering done much quicker. You still will want scissors to perfect it, but trimmers will do the bulk of the work for you.