If you're taking up knitting, you'll soon learn that serious knitters use a lot of yarn, which means buying a lot of yarn. Buying cheap, inferior yarn isn't a good way to produce high-quality shawls, socks or sweaters; if the cost of yarn becomes a problem, buying wholesale instead of retail is a better choice for finding quality yarns at a price that won't bust your budget.
Check online. A Google check on "buying yarn wholesale" will give you multiple websites selling yarn: wholesaleyarnstore.com, numei.com, jagger-yarn.com and many others. If you know other knitters, ask them if there are sites they'd recommend, or if there are dealers they'd warn you to stay away from.
Decide what you need. If you're knitting a sweater, what colour and quality of yarn are you looking for? Or do you simply want a variety of high-quality yarn that you can then knit into different projects?
Comparison shop. See which websites have the yarn that you want, and what it will cost. Make sure to find out how much you have to buy to get a wholesale rate, and what the shipping fees are. The cost of shipping could turn a good deal into a very expensive buy.
Place your order and watch the results. If delivery speed or yarn quality isn't what you were promised, let the website know. If they don't resolve your complaints fairly, don't use them again.
If you patronise a local crafts or yarn shop, ask what sort of bulk discount you can get as a regular customer. If they're willing to cut a deal on large purchases, there'll be no shipping charges, no wait for delivery and you'll be supporting a local business. The best approach is to ask to speak with the assistant manager or manager, and ask how much yarn they have in stock of the particular kind needed for the project, and if they offer discounts on buying yarn in bulk. It's important to buy all yarn for a project from one dye lot to make sure that the finished product will have uniform colour. Even whites and light colours can have subtle differences in dye lots. The dye lot is printed on each label that wraps a ball of yarn.
If you buy yarn online, follow the same precautions as with other online purchases: Make sure the website is secure before you give them information; check your credit card statements for any unauthorised purchases afterward; don't send credit-card information over open, public wireless systems where hackers can pick it up easily.