I bought mine at the REI in Fairfax.
I took my drysuit and socks with me to the store. Got the sizes I wanted to try on, sat down and removed my socks and shoes and put on my thin wool socks with my 150gsm Thinsulate socks, then put just my feet and lower legs into my drysuit. Then tried on the paddle shoes.
My normal shoe size is 10.5. My NRS paddle shoes are size 12.
I have never had any luck using the same fins for wetsuit and drysuit. Not only do I need one size larger fins for drysuit (XL vs L), I need fins that are negatively buoyant, as well. In a wetsuit, I use neutral fins for most diving - except back mount doubles, where I use fins that are a bit negative, but not as negative as my drysuit fins.
EDIT: Doh! You're probably in Washington state, not DC....