Quote from thisisbabak:
If you don't have access to a Bloomberg or Reuters terminal then I'd suggest looking at page 1204 of Fabozzi's "The Handbook of Fixed Income Securities" for a formula for implied repo rate (do a search for it on Google Books). Next get conversion factors from CBOT.com or calculate yourself, followed by bond pricing from Yahoo Finance (search for eligible Treasuries using the bond screener) or other source.
If you have access to term repo rates, then you can improve your analysis by considering actual term repo rates. The CTD is the bond with the smallest absolute difference between the implied repo rate calculated above and the actual term repo.