Below are listed several online repositories of data:
http://astro.estec.esa.nl/Hipparcos/research.html
This site contains various search tools
available for querying the Hipparcos and Tycho Catalogues.
ftp://cdsarc.u-strasbg.fr/pub/afoev/
This is visual photometry of hundreds of variable stars, made by
amateur-astronomer members of the French association of variable star observers.
The files are arranged by three-letter constellation name (e.g. ori for Orion)
and, within those files, by variable star name (e.g. alpha for alpha Ori or
Betelgeuse).
http://schwab.tsuniv.edu/papers/aj/youngsuns/youngsuns.html
This is differential
photoelectric photometry in V or y (yellow) or in B or b (blue), of stars like
the sun. They vary by up to a few hundredths of a magnitude in brightness, on
a time scale of a few days, due to rotation of their star-spotted surface.
http://schwab.tsuniv.edu/t2/sarv1/sarv1.html
This is differential photoelectric photometry
in v (visual), r (red), or i (near-infrared) of red giant stars. They
vary by up to a magnitude or more, on time scales of 10's to 100's of days or
more, due to pulsation.
http://www.aavso.org
The American Association
of Variable Star Observers (AAVSO) provides a large quantity of data specifically
on variable stars. They do not yet have all of their data available for download,
so you must search around for your particular star, and possibly request them
to contact you with the data you desire.