You are sooo lucky!!
I've used this site a lot to find local places and to find specialty shops for those difficult to find parts:
For instance, I found California
Mustang the best place to get paint in the can for vinyl, etc.
If you live in a major city, there are several mustang shops in your area that you can go to that will give you personal service and advice. Check your local phone book. I have three in my general area and visit each one when I'm in the area.
For the items you are looking for, most web sites will carry them. Since these parts are manufactured by only a few manufacturers and sold by many distributors I found that buying from the one closest to you is what you need to do in order to save on shipping costs. I use California Mustang because it's only 30 miles away and I really like dealing with them, I can get the same part shipped to me a lot cheaper than buying from National Parts Depot or Paddock. The web site I gave you will list distributors nationally so pick one that is close to you.
I would also suggest you contact and join a local Mustang club. These people are a great source for used parts, technical support and events to show your mustang off at. They have great picnics!
Subscribing to Mustang a mustang magazine such as Mustang Monthly is also a great source of "how to" projects and restoration tips.