Monthly average rate of discount, 3 month Treasury bills, Sterling Vs 1 Month
Answer to your first question
Take a look at what Aswath Damadoran uses for his empirical work. He's been teaching applied corporate finance for a little while at NYU. These are his slides.

Answer to your second question
If you have access to any of the following, you should have no problem getting access to any economic and financial time series.


Datastream
Bloomberg


Also try FRED database