I was in the market for a LBD (Little Black Dress). I was in full male mode at the time. The first store I walked into didn't have one... So, my next store was a Dressbarn. There I meet Theresa, and she was fabulous. She sized me up right away and got me a selection of LBD's to try on, wow I was in Fem heaven. I thought that typical male I'd be on the plus side, but I'm shopping the Misses side now thanks to Theresa. I love shopping DB. Last weekend I even stopped in just to say hi, as Theresa had been out during my prior two visits. She was there and we had a nice talk even thought I didn't purchase anything.
I also love to shop Target, specifically for my panty/bra/hosiery but also have a sweater-dress from them as well.
Then there's the Red Liz Clairborn dress that I picked up from JcPenny's along with a black cardigan sweater. I know that most department stores I'm not the first TG person in there. Depending on location, (some people just can't leave their narrow views to themselves) I've been in the regular female dressing rooms. Funny, they look pretty much like the Men's dressing room (actually pretty much the same with the women's room too: once you've seen one cubicle, you've seen them all).
So, I recommend you go to a department store during slow hours, maybe just after the lunch hour 2-4pm on a week day would be a prime time, they can peg your size to a couple of options, and you can try them and find the sizes that work for you. While there are some variations between the different manufacturers, if you're a 20 it'll typically be between an 18 or a 22. So, getting sized and trying a few on will get you an idea of what works and what doesn't.
Happy shopping. I hope you find lot of items you love to wear.
-Alana