Shops in Kathmandu
Kathmandu's tourist district of Thamel (and to a lesser extent, the city of Pokhara) is chock-full of shops offering every type of outdoor gear imaginable. Many of the items are under the guise of the major outdoor brands. However, the issue is not quantity but quality: virtually the entire supply is cheap counterfeits made from the lowest quality materials. A shop in the heart of Thamel sells fake logos for all the major outdoor brands and everything down to water purification tablets is likely to be counterfeit.
What does this mean for trekkers? It's perfectly reasonable to buy non-essential items that won't affect the outcome of your trip. T-shirts, shorts, sandals, sunglasses, and similar items are generally okay to purchase. You are taking a much greater risk by buying trekking or camping equipment that requires specific performance. Items to be avoided include hiking boots, cold weather clothing, sleeping bags, tents, and water purification (for cheap gear options see the "Renting Authentic Equipment" section below).
Renting Authentic Equipment
A good money-saving option is the small warehouse shops in Kathmandu that rent real expedition equipment. These establishments recycle genuine cold-weather gear and specialty items that are only needed for Nepal. You may want, for example, a heavy down jacket and down pants for crossing a high pass. You can rent these items for less than a dollar per day and return them when the trek is finished.
To locate a good rental shop, the best option is to ask your guide for a recommendation. Many of the guiding companies have business relationships with legitimate rental shops and they will help you find a quality place. Once inside, it shouldn't be too difficult to distinguish whether or not the equipment is real—inspecting the logo is a good place to start. Check the items for good fill as older items can be light on down. And don't be afraid to ask for more options.
Buying from the Big-Name Retailers
Due to popular demand, the big-name outdoor gear manufacturers such as Mountain Hardware and The North Face have established a presence in Kathmandu. The stores are on Tridevi Sadak, a quick walk from downtown Thamel. There you can buy anything that was overlooked before departure and it's guaranteed to be the real thing. The stores may be slightly cheaper than their Western counterparts but not enough to make a difference (around 5% cheaper with limited styles).