My students always inquire as to where I shop for my art supplies. Unless I need supplies in a hurry, online is where I get the majority of my supplies. It is so easy to comparison shop and get the most for your money. I always check on shipping costs as that will greatly effect your final price. Companies will always send you their catalog for free if you request them, so you can easily see what is offered before you go online. Here are some of my recommendations (not in any particular order):
New York Central Art Supply - This company has many hard to find and unique items. They also have specialty catalogs that you may request.
Terry Ludwig Pastels Great luscious colors and wonderful personal service.
Some great plein aire easels and pochade boxes are offered by varying companies. I recently bought an EasyL which I adjusted to my needs. I created 2 boxes which fit within the main box. One box holds my pastels, the other holds a palette for oils. I alternate the box inserts depending on what my desired medium is for the day. You can see the box with the pastel insert in the photos of my demo at the Pastel Painters Society of Cape Cod. I will include a picture of the oil palette in the next few days.
I purchased my EasyL here:
Judsons Art Outfitters is currently having a seconds special on some of their boxes.