Sources of Data

Below are listed several online repositories of data:
This site contains various search tools available for querying the Hipparcos and Tycho Catalogues.
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).
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.
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.
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.