How can I find statistics for the top 100 bestselling books in the
field of youth ministry, for the period since 1993 through the
present?

The Evangelical Christian Publishers Association
(http://www.ecpa.org/ECPA/bestsell.html) provides bestseller lists in
a variety of ministries (including youth) by month from January 2002
to March 2004. The lists are compiled based on information contained
in the ECPA STATS report. The report has not been around long enough
to trace bestseller lists in 1993. Hope this helps.

STATS Homepage:
(http://www.ecpa.org/2003ChristianPublishingTrendsReport.html)

STATS Login:
http://ecpa.irtnet.com/