So what about choosing your patterns? As Bane of GIY: Goth it Yourself says, "ignore the fabrics and colors used in the pictures or drawings on the envelope." Instead, look for the overall design. Look for where the item falls on the body, what the shape is, what details are sewn in. The line drawings will help you with this exponentially. Don't let the styling of a pattern push you away. Almost every item of clothing can, if done in black, work with a Goth wardrobe. Well, I say almost. I'll eat my stripy socks if someone can make the jumpsuit pattern from this blog post into a Gothy ensemble.
On that same note, finding fabric is a whole other kettle of bats. Your local fabric store is a great options around Halloween time, and there are also online retailers. If you're feeling particularly bold and artistic, Spoonflower allows you to make custom fabrics from your artistic designs. Once you have your fabric, and any other details you choose (buttons, lace trim, ribbons, grommets, chain, etc.) you'll be ready to turn any pattern into a great Goth garment.
So what patterns to choose? Well, unfortunately, there's no blanket answer I can give to that beyond whatever you like. Simple skirts are probably one of the easier patterns to start with if you're new to sewing with patterns, but beyond that anything goes. And lord knows there are patterns for anything you can dream of. Jackets, skirts, legwarmers, trousers, underthings, petticoats, blouses, t-shirts, coats, and dresses, and that's just the tip of the iceburg. All it takes is a little imagination, and the above reminders about following the lines rather than the styling, and most patterns will fit a Gothy wardrobe.
Some pattern shops have two different costume sections. The first is for historical costuming, and the second is for Halloween costumes. I recommend you check both, but note that the Halloween costumes will probably be sized more generously and be looser because most people don't want to be miserably uncomfortable in their Halloween costumes that they'll be wearing once a year. Since historical costuming patterns are used by reenactors who wear their outfits more frequently, you might see more true sizing in them. But, of course, it depends on the pattern.
When finding your pattern, it's necessary to know your keywords. It can be intimidating to find patterns and sift through all the pattern ID numbers, but luckily many pattern shops online offer descriptive words to assist you. Unfortunately unless you're at an awesome seller, "Goth" isn't going to be one of those keywords. So, what else can you search for? Romantic, medieval, Steampunk, Victorian, Vintage, historical, gown, vest, and corset are all excellent options, though of course it depends on your aesthetic. Still, you're more likely to find costumes for a Romantigoth, Victorian, Lolita, Vampiric, Medieval, etc. costume than you are for, say, Cybergoth.
Luckily, you're not obligated to stick to the pattern all the time. If you have a little bit of draping know-how, or are good enough with sewing to make some guesstimations, you can make alterations to the pattern. Hate the collar? Ditch it and make a new one. Pattern is too big in the bust? Make a small bust adjustment. Think the ruching is god-awful? Draft a pattern piece to replace it. It's definitely not impossible.
So, where should you buy patterns? Perhaps the most popular pattern companies that I know of are Simplicity, McCalls, and Butterick. If you're looking for vintage patterns, Etsy and eBay are your best options.
How do you Goth home sewers out there source your patterns for Goth clothing?