Need authoritative source for historic Bitcoin conversion prices from all exchanges
All historical data from any exchange available at Bitcoin Charts: Markets API


  Historic Trade Data
  
  Trade data is available as CSV, delayed by approx. 15 minutes. It will return the 2000 most recent trades.
  Format:
  
  https://api.bitcoincharts.com/v1/trades.csv?symbol=SYMBOL[&amp;start=UNIXTIME]
  
  Returns CSV with columns: unixtime,price,amount 
  
  Example
  
  Latest Kraken EUR trades:
  https://api.bitcoincharts.com/v1/trades.csv?symbol=krakenEUR