The gothic thing sounds like a good call: the goth websites cover all sorts of sizes.
This is mostly hearsay, but Honour (honour.co.uk) and uber (uberkinky.co.uk) both do some plus size stuff. (When I was dating a plus sized fetishist she was definitely taken with Honour, and had got some nice stuff from there.) I'm told that there is stuff on ebay in plus sizes as well, if you know what you're looking for, rather than browsing. Freak Clubwear might be worth a look as well.
Beyond that, no idea.
On the level and looking for a square deal.