for high impact sports like running, it's best to wear sports bra.
regular bras with elastic straps may caused breasts to sag.....too much of "bouncing" i think.
look for sports bra with hooks.
u can read more in many websites on sports bra...types, sizes etc.
just google them.

for large sizes, u can try to find in Marks & Spencer.
they have sizes up to DD cup.
ask the sales assistant to measure ur gf's size.
that's where i got mine.

i heard triumph also has sports bra.

good luck