The advice which is always given in publications such as the Writers' & Artists' Yearbook is that unless you (or someone you know) can produce illustrations which are
better than the ones which are produced by the people who're employed directly by publishing companies, you should simply submit your text to potential publishers and leave them to find suitable illustrators.
However, if you're determined to team up with an illustrator before approaching a publisher, look through the websites of the illustrators who are members of SCBWI. (Use the alphabetical index on the left of the page and then click on links marked with an asterisk, which indicates that the member is an illustrator):
http://www.scbwi.org/links/mem_links.htm
Chris