Shannon:

Traditionally it's 0.1% azide (1.0g sodium azide in 1.0 liter of 1%
bSA).  It isn't a very effective bacteriocide, though, and disposing of
it legally is a nightmare.  Interestingly, it is biodegradable, with
some fungi using it as an energy source.  Go figure!
We preferred to use Kathon, which we got from Rohm & Haas, although it's
now available from one of the minor lab-chemical suppliers.
Good luck,

Adrian Leek