I am very much impressed with a stock icons vendor IconDock. They have limited but very nice icon set. What is special about them is that they sell individual icons too.
Their website is very well designed.
Here are some of their limitations (as of today):
- They sell only 16×16, 32×32 and vector sets. It is developer’s responsibilities to use a vector image editing software such as Adobe Illustrator to edit their vector sets to any size they want. Not all developers (especially uISVs) own a license to Adobe Illustrator.
- They do not design custom icons. If you don’t find all the required icons in their icon sets then you are stuck. You can get the additional icons done by them.