It can be difficult to trace various editions of inscriptions, but fortunately help may be found:
Supplementum Epigraphicum Graecum
Of course you should consult SEG via the indexes. For instructions click here.
The concordance site Claros allows you to find out quickly whether a particular inscription has been republished elsewhere. Note that the site may be slow or unresponsive, probably due to too many visitors. It is often best to try again later.
The University of Münster offers an on-line concordance for IG to IG and IG to SEG (Also accessibe via IG site)
A concordance for the inscriptions in Rhodes-Osborne can be found here.
Click here for Concordances for CIL with earlier editions and addednda and corrigenda