I just posted this photo to the Dolly Kei outfit thread:
http://tokyo-fashion.tumblr.com/post/8257611918/dolly-kei-girls-in-tokyo-wearing-all-white-outfits
I know you said that you don't like online shopping, but honestly that's going to be your best bet for finding Dolly Kei pieces. If you are worried about sizing, just ask someone to help you take your measurements. Most vintage sellers will only give your the chest, waist, hip and length measurement, and that's all you really need to know.
Here's a tutorial on how to measure yourself, although it's more detailed than you probably need: http://www.burdastyle.com/techniques/how-to-measure-yourself
The reason why I stress the value of knowing your measurements and being willing to shop online is because of things like this:
http://www.etsy.com/search/vintage?search_submit=&q=white+prairie+dress&view_type=g allery&ship_to=US
As you can see, there is tons of choice and variety available. You can easily use most of those dresses in a beautiful dolly kei coord.
You could always try searching in your local thrift/vintage stores, but it will take you longer to find something and be more hit&miss.
Good luck!