My line is currently stocked on
www.OneAnthem.com and
Vice Wears - Cutting Edge Clothing
both shops that exist soley online. For both of the companies they contact me first to stock the line, however it should be fairly easy to get into contact with an online boutique for stocking your line.
A lot of shops have contact pages that should have information for brands that are looking to have their lines stocked at a specific site. If not this a general email address for support is almost always on a site especially an online buisness.
If the site has neither of these you may want to rethink doing business with them in the first place.
Conducting business with an online retailer means that communication is going to be of the utmost importance. If you cannot even get into contact with them to consider stocking the line than when it comes time for you to get payed for your first shipment than you could be in an even worse position.
Hopefully I've been of some help...