Arkham, if you like shopping online, definitely check out Retroscope Fashions for new gothic/victorian/steampunk-ish separates (www.retroscopefashions.com). They cater to many sizes, so fit shouldn't be an issue. The other online retailer I'd suggest is www.historicalemporium.com. HE is a dealer of several different brands that produce period clothing designs as new clothing. Again, there's a wide range of sizes.
If you want to shop locally, Bedlam Bedlam might be a good option for you (on East Pike). Mishu could be another good place to look - they've got a store in Fremont, and I think they've got another location on Cap Hill, but I haven't been there.
If you like strolling the online hall of Etsy, there are about a bajillion people making their own steampunk clothing and accessories. If you can account for shipping time, and you have a good idea of the look you're going for, that would be a fantastic way to go as well.