Some of the popular online stores for Gothic clothing are:
http://www.ebay.com (search for Goth)
http://www.gothauctions.com
http://www.gothicauctions.com
http://www.gothicbeauty.com
http://www.hottopic.com
http://www.kinkyangel.co.uk
http://www.findcoolclothes.com
http://www.lolita-kisama.com
http://www.lip-service.com/
http://www.goodgoth.com/
http://www.velvetgarden.net/
http://www.x-tra.de/english/index-2.html
http://www.rosemortem.com/
http://www.thedarkangel.co.uk/
http://www.jeannienitro.com/
http://www.phaze-clothing.com/
http://www.moonmaiden-gothic-clothing.co.uk
http://ipsofacto.fateback.com/
http://www.dracinabox.com/
http://www.crimsonempress.com/