Yoga, like many other practices, requires quality comfortable clothing. Not so much because of any ancient Yogic tradition, but more because it is much easier to concentrate on one's Yoga practice when you're not always thinking about what you're wearing! In any practice that requires movement - Yoga, Tai Chi, Martial Arts, etc., it's important that your apparel move with you and not be so tight as to restrict your movemnet, or circulation - and also not be too loose as to get caught up, twisted, or cause chafing. Yoga apparel is, of course, has it's own unique requirements - often dending upon the style of Yoga you're practicing.
If you currently study with a Yoga instructor or school, observe the type of clothing your teacher wears. The school may also recommend or sell a line of clothes for Yoga, or at the very least can point you toward a Yoga clothing store or manufacturer. The same goes for Yoga mats and other training equipment. It's always a good idea to ask those that practice very often and have put their cloths and other training gear through their paces. If you're working up a real sweat, make sure you buy Yoga clothes made of fabrics that breathe, wicking the sweat away from your body. If you normally sweat a lot during your Yoga practice, those types of fabrics also tend to dry out much faster when you're workout ends - unlike cotton, which tends to stay damp for a while after your practice session ends. Watch also for fabrics that can cause skin irritation and chafing, if you've ever had an adverse reaction to vertain types of Yoga wear or other workout clothes.
A carefully chose collection of Yoga apparel will server you well for years and will help you keep your mind on your Yoga practice and away from your clothes.