Is seems NOAA actually has geomagnetic indices here
principle you could download it and even look at a GIF animation, however I always get the error message:
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Anyways I haven't even found an official definition of the aa index. The aa index starts in the 19th century, while the K and ap indices only in 1932 according to the NOAA page.
It's also not explained here.
The aa.doc in http://isgi.latmos.ipsl.fr/source/indices/aa/
doesn't say at which time of the day the three-hourly measurements start.
all this is annoying.