I know a few sites that might be relevant, although to be honest I'd recommend you reconsider on buying a gothic tophat...
I used to live in Camden Town, London back when it was considered the "goth capital" of the UK. In recent years it has become a bit more of a tourist attraction, and less of a great place to go clothes shopping.
Judging by their online offering though, they have started catering to a more general and less "goth" audience, probably because it's pretty difficult to run anything gothic and still make a profit these days.
Most goth nights etc... Have closed down are have forced to become more mainstream, although the lineup for http://www.alt-fest.com/index.html looks pretty cool!
Anyway... I digress!
Back to the top hat. Most goth stores don't seem to stock interesting stuff in their online stores, but I would recommend eBay or Etsy for what you're looking for.
They're a bit cheaper than most larger retailers, and some of the handmade one-off pieces look pretty cool.HOWEVER...
The reason I think it might be worth reconsidering is that, if you look at most of the "gothic top hats" on sale, you'll find that a lot of them standard top hats with some "added goth bling" in the form lace, synthetic purple roses, feathers, or whatever gothic styling has been added on to make them more aesthetically sinister.
Now I'm not saying that the ones on offer don't look great, but often taking the DIY approach will set you back only a fraction of the cost of buying a pre-customized top hat.
You can pick up a top hat from most vintage shops these days, or buying a plain black top hat from an online retailer shouldn't set you back too much.
BUT MAINLY, I think it's kind of a fun thing to do, visiting the haberdashery section of your local department store and stocking up on lace, velvet, and other goth-friendly materials can be a good way to spend an afternoon for some :)