Medicine bags are found at vendors who sell shaman potion-making supplies. They are 8-slot bags weighing 0.4 stones and cost roughly 1.5pp each. The cost will depend on faction and the buyer's charisma. The tradeskill-made Reinforced medicine bags are 10-slot, weigh about 2.5 stones, and does offer 10% weight reduction value and costs roughly 30-50pp each from tradeskillers. Of course, you can build your skill a little and farm your own goods and make them yourself.