International Monetary Fund (public site):

For background information (what are special drawing rights, what role do they play, how are they determined), review the International Monetary Fund's factsheet.

The IMF's public site also includes a list of SDRs per Currency unit and Currency units per SDR (last five days) as well as an archive of that data back to 2003.
 

IMF eLibrary (HarvardKey access)

• Once in the database, click on IMFData at the top of the page.
• Click on Datasets
• Select International Financial Statistics, then click Query.
• Enter a date range, and limit to specific countries if desired.
• Under Indicator, search for SRD and add any indicators you'd like.  Click apply.
• You may download your results to Excel.

Bloomberg:

Type SDR and hit the yellow CRNCY key, then hit GO. Use the menu of options for the SDR unit of "currency" to locate information.