GOES18 image.

GOES satellites carry onboard a Space Environment Monitor subsystem that measures X-rays, Energetic Particles and the Magnetic Field at the spacecraft.

GOES Space Environment Monitor

Primary access to GOES SEM data is through the interactive tool -- SPIDR.
./data ./avg ./YYMM (i.e., ./2008)   1 min & 5 min averages, ASCII tables, view naming and format.
  ./full ./magnetometer ./YYMM Full resolution, 3s values, FITS binary tables, 1974 - 1996.
    ./particles ./YYMM Full resolution, 3s values, FITS binary tables, 1974 - 1992.
    ./xrays ./YYMM Full resolution, 3s values, FITS binary tables, 1974 - 1996.
  ./plots ./YYMM   Daily & montly plots of all data in time series used for quality control, pdf format.
./docs ./avg     Description of 1 min & 5 min average data files.
  ./full   Description of full resolution data files.
./software       C Code to convert full resolution files to ASCII tables.

In addition to the http access shown above, this same directory structure can be accessed though ftp (ftp://satdat.ngdc.noaa.gov/sem/goes/).

This list of energetic particle events is prepared by the Space Weather Prediction Center
The Space Weather Prediction Center maintains a historical status page for GOES SEM data.
GOES Data Books
The GOES Data Books provide a good overview of the satellite platform and various instrument packages.
GOES-8 - GOES-12 Data Book
GOES-13+ Data Book
Historical data quality issues are discussed.
The NORAD ortital elements for GOES are available from CelestraK.
ERL-SEL-42: Space Environment Subsystem, 1975, R.N. Grubb
ERL-SEL-48: Solar X-ray Measurements from SMS-1, SMS-1 and GOES-1, Information for Data Users, R.F. Donnelly, et al
ERL-SEL-56: SMS-GOES Soft X-ray Measurements, July 1, 1974 - December 31, 1976, R.F. Donnelly
ERL-SEL-62: A Summary of Solar 1-8 Å Measurements from the SMS and GOES Satellites, 1977 - 1981, S.D. Bouwer, et al
Monitoring space weather with the GOES magnetometers, SPIE Conference, H.J. Singer, et al
Comment on the use of GOES Solar Proton Data and Spectra in Solar Proton Dose Calculations, D.F. Smart & M.A. Shea.
Here you can find several custom plots. There are also links to the Spacecraft Anomaly Database files.
You can view these data in real time by visiting the Space Weather Prediction Center.

GOES SEM data can also be ordered on CD-ROM or DVD-ROM for a nominal charge.