thrift shops are great sources if you have good ones by you. most of the ones by me have dried up (i had a hard enough time just finding a plain white dress for a mummy costume if you can believe it). thrift shops will be the best resource for steve as far as finding a nice cheap suit (if that's the look you're goin for).
if you're trying to stay on a budget, i recommend finding something you like online and then searching elsewhere for a similar style that's cheaper or even modifying something you already have. i could give you a jillion websites, but you're not going to find much cheap. shops like daddy-o's carry gorgeous dresses, but you'd be lookin at throwin down anywhere from $80-$150 for a dress.
ebay is also a GREAT source for finding vintage and re-pro's.
keep in mind that your accessories will help bring the look together (waist belt, jewelry, hair-wear, parasol, neck scarves, hairstyle, etc).
pm me if you find yourself having a little trouble and i'll gladly help ya out.