There are a couple of popular ebay sellers (check through some of the other links for the popular ones). The ones I have bought from places other than arda run small for me.
You can also easily adjust the size. If you look at the inside of the wig - you will see the wefts sewn in (the lines of actual hair). If you take a needle and thread and sew together two adjacent rows of wefts - you will make the wig smaller. You may need to sew a few rows together in different areas. It won't show on the outside at all.