go to www.tinytotdressshop.com, not sure how much your wanting to spend but theres an adorable blue dress on there listed under easter dresses but it could be used for a cute picture. Also www.cutieclothes.com it actually has a button to click on just for blue dresses - I hope this helps.
you can try etsy.com it is a site where many moms sell handmade items. I have bought alot of clothes and such there. you can find some really original CUTE toddler clothes, shoes, just about anything. search blue dress 2t and you'll get pages of a great variety of dresses