I've dabbled in both punk & goth styles, but, that's in the past for the most part. I still have lots of band/alternative hoodies in my closet that I wear, and some tops, but, my plaid bondage pants days got left behind with the thinner me.
The first obvious answer would be Hot Topic & Torrid.
I also really like Obey clothing, I have a wicked hoodie from them that I got ages ago, and it's worn all to ****. Not honestly sure about how large their sizing goes, but, it's still worth a mention.
http://shop.obeyclothing.com/
I also did a quick Google search for "punk plus size clothing" and was taken to this page, which seems to link to other sites that are "Punk - Plus Size":
http://www.clothing-underground.com/PUNK_plussize_1.htm
Hopefully this was somewhat helpful, and I'll be sure to keep my eye out for you while I browse around.
Oh, also - EBAY!
