The mother of all supply houses:
Carolina Biological Supply. Their online catalog has everything from
insect pins and "vital dyes" to pre-made microbiological media, incubators
and microscopes.
Other supply houses to try include
vWR and
Fisher Scientific, though keep in
mind both companies permit orders from entities with institutional
accounts (such as your school or university).
Microbiological Medial: Other places to try include
Remel and
Oxoid (Fisher Scientific).
Sigma-Aldridge Chemical Company:
They require you to open an account, and in general only accept
orders from individuals with institutional affiliations. However,
their entire catalog is online and may be searched.