Just thought I'd pop in here and agree with everyone else. Charity shops and jumble sales are brilliant, but bear in mind if you're squeamish, 99% of very old stuff donated to these sorts of things is Dead People's Clothes. Doesn't bother me one bit, but I know some are a bit twitchy about the idea.
Most of my really old stuff comes from Fleabay. Good quality high waist wool trousers tend to sell for around £10-£20 as they appear to be virtually indestructible (post Apocalypse, the cockroaches will roam a land strewn with old wool suit trousers). But be careful of sizing and pay attention to measurements given in listings, they've usually been tailor made for their original owners, so they sometimes have rather odd waist/inside leg ratios. Also, be prepared to do a bit of mending/adjusting on older clothes, they're rarely in 100% perfect condition, and don't fear actually washing certain things! Just be very careful, use cold water in a bathtub with a gentle liquid detergent, and keep them flat at all times. Summer is a great time to do it as you can dry them within a day from dripping wet. Don't do it in the winter, they'll stay damp for ages and get that smell... Some old clothes come with decades of unseen, ground in, filth, and you don't want to walk around with some deceased person's skank hiding amongst the fibres of your collar. Xp
Otherwise, learn to make stuff. It's the best way to get what you [i[really[/i] want, but not always the cheapest option if you choose a nice fabric (waistcoats only need half a meter for the front, but a gown with a huge skirt can be 8 meters or more, plus what you need to make any petticoats it requires, ect. and that's when it really starts to get pricey).
