The local chapters of Parents of Multiples (POM) and Mothers of Multiples (MOM) all run sales in both spring and fall. You can visit their web-sites to find the dates of the local chapters. The ones that I know of are:
MCPOM (Montgomery County POM):
http://www.mcpom.org/
CAMOM (Columbia Area MOM):
http://www.camom.net/index.shtm
AMOM (Annapolis MOM):
http://www.annapolismoms.org/Moms_Club/Welcome.html
NVPOM (No Va POM):
http://www.nvpom.com/
These 4 chapters make sure not to overlap their sales, so you can go to 4 sales in the Spring and 4 sales in the Fall (if you wanted). Check out each of the web-sites for when and where they'll have their consingment sales. Most of the clubs use the proceeds from the consignment sales to fund most of their club activiities, so they are big and well advertised.
My feeling is that my twins will have to dress and use mostly gently used clothing until they are old enough to tell me otherwise, what they want. Then we'll talk about it.