We have in my area what's called Discovery Shops. They are second hand stores run completely by volunteers. All proceeds go to the American Cancer Society.

I would take a trusted/honest friend with you - someone who will tell you what looks good, what is flattering on you.

And my word of advice would be - don't pay any attention to the sizes. If you look good in a size 12 then wear that. Even if you want to be in a 10! Nobody knows (or cares) about those digits on the tag, what they will notice is how good you look! (and wearing proper fitting clothing will always be more flattering than squeezing into a smaller size)

Happy shopping!