Starting an online boutique has never been easier than it is now. There are many legal loops you will need to go through to order wholesale products from vendors. See the cost of you state's seller's permit (if a cost) here.
If you haven't yet got your permit but are ready to start selling, there are tons of websites dedicated to bringing wholesale pricing - similar to those on websites.
This is great if you also have limited capital and do not want to purchase your clothing in sets of 5 or 6. You know your audience best. Your audience may order Mediums more than any other size. Purchasing through these websites will allow you to purchase multiple mediums vs. only 2 per set, which you would find at wholesale websites.
There are pros and cons buying clothing for your store this route, with the pros outweighing the cons. You could stock your entire store this way without ever having to use a wholesale vendor website.
You do not have to purchase a min quantity
Does not require a seller's permit (sales tax permit, wholesale license) to purchase
Often will find trendier items
Able to see all clothing on a model or how-to dress it
Often has more details on the item that you can include in your product descriptions
Affordable to buy blank apparel in bulk and start a brand
May run out of stock without notice and not required to restock for you
Clothing tags may or may not have a brand on a sewn-in tag (does not matter at all to some customers)
Shipping may take up to 2 weeks to arrive
Below are my favorite wholesale-like companies that do not require seller's permits.