Though there is little chance that I will stray from Lululemon, it is nice to see that other yoga apparel options are becoming commonplace. In addition to Old Navy (you can find its line of yoga clothing here -- pants start at $16.50, some tops are on sale for $10), JCrew sells yoga clothing online and through its catalog (not in the store). I recall a rather robust line, but its website is a bit sparse at the moment: see here.
Other brands I like include Be Present, which has a line of items made from earth-friendly fabrics, and prAna, which makes durable, fashionable yoga clothing and a large line of yoga-inspired items that are slightly dressier and less I-am-running-to-and-from-the-yoga-studio that Lululemon's. Of their new spring items, my favorite is the jewel halter dress (pictured). Made from organic cotton and spandex; it comes in the pictured black, white, and a deep brown. $75; find it here.
Both Be Present and prAna sell their merchandise via their websites. Though most yoga clothing and accessories are vegan, it is always good to check; for example, Lululemon now sells a leather bag (eeek!).