Hi Andy,
Clothing brands even large ones often source through sourcing agents as it's not possible for them to have a presence in every country. Buying agents provide the quality control at factories as well as negotiating prices and dealing with customs. In fact most of the factories I have been in contact with get the majority of their orders through independent buying agents. Most of these buying agents are small to medium sized companies but have dealt with major retail chains and clothing brands.
I don't intend to approach really large chains until I build up a track record.