You could try
http://www.studysoccer.com.
It has a searchable database of goaltimes in matches. You could do a search to count all matches where there is 0 goals after 0 minutes, then search for all matches where there is 1 goal after 15 minutes.
Divide the 2nd answer by the first, then bob's your uncle, that's an estimate of the percentage of matches where there's a goal in the first 15 minutes.