Shop the stash before you shop the shops.
Formal-ish: scarves, shirts, vests, old evening wear, old best suits, 'tai chi shirts', things made of satin or brocade, formal shoes, non-terrycloth bath robes without pockets.
Casual: tees with oriental-inspired prints, army boots, cargo pants, jeans or twill jackets, less formal brocade items.
Army surplus: khaki green or desert beige clothing, belts, pouches, straps, boots.
Thrift store: evening wear, shirts, vests, leather, belts, shoes, gloves, hats, clothing to alter or harvest for fabric.
Mundanewear shops: standing collar shirts are somewhat fashionable for men and women atm. Some tops come in a mock wrap style that's a nice fusion of eastern and western.
Street market, Asian shops/webshops: brocade items, 'tai chi' shirts, fans, parasols, sarees, costume jewellery.
Silk painting supplies: silk scarves for cravates or to pair with evening wear. Plain undyed ones can be surprisingly cheap.
Workwear shops: cook's tunics have standing collars, sometimes wrap around and can come in colours.
This close to an event, I'd say shop or shop the stash as much as you can. Consider if you can alter what you have. For instance, fitting something more closely may make it look more formal. You can also sew trim around a (men's or women's) v-neck to make a mock wrap shirt.
Taking in a bought shirt (also works for vests, etc. Check out more of their movies for spats etc.)
Making a faux-cravate out of thrift-shopped ties
Some more thoughts from another larper about Firefly larp costuming