I've found the best time to thrift shop is during October in the Halloween section. Most thrift stores save pieces through out the year just for this time. I've found pieces from the 1940's, vintage beaded flapper dresses, 1950's tulle dresses...things you dream about for next to nothing.
I look for vintage dresses like these for special occasions. This would make an amazing wedding or reception dress.
I always look for fabrics regardless of the silhouette. Think velvets, beaded garments, lace...the silhouette may need work, but you can't get the fabric anywhere else.
Bows and lace...These dresses were from the 1950's. The style and fabric usually give you a vibe of what era, but the tag can have a date. These were put in the Halloween section ranging from $3 -$5...
I always go to the linen and table cloth sections for my fabrics...I already see some projects in there.