fresh can be found at
Lim Bangkok
Manphong
Kowloon
Uni
and I am sure there are tons of other places
I normally get mine at Lim Bangkok as they supply most of the SE Asian restos and the turnover is good
check the bases/root for mold
to use: Split and use as a skewer
mash with a hammer or cudgel for soups, stocks or tea
the frozen prechopped stuff is used mostly in stirfries but loses a bit of flavour
Kowloon Market (720 Somerset W) has these freezers in far end at bakery side of the store, and they have these small containers of frozen minced lemongrass. It is extremely helpful as it is sometimes hard to source lemongrass in 'nepean' area of town... and since it's already minced it's very easy to use. They also have the fresh lemon grass, not sure if it's seasonal.
zymurgist