When I was a 34B (many moons ago), I really loved long-line/bustier styles from department stores - I always felt like convertibles or bandeau style strapless bras would slide down, but I felt super-confident about the lift and support of the long-line style!
I've forgotten how they're sized (been a while since I wore anything requiring a strapless bra) but I seem to remember having to try on at least two numerical sizes to gauge how they fit my lower ribcage vs my upper ribcage (remember, they're long) and I definitely recommend trying a few on! I think I wore department store brands, like Maidenform and Playtex, back in the day!
==========================================
Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle