Best way to find references for statements in the thesis
Although the question is a bit vague, I try to give you an answer from my experience. There are often statements that you heard somewhere or which are "obviously" true, but you need a reference. 

My best bet is to search google scholar for a variety of different combinations of the terms or suspected authors. If you find some source stating the claim (or something similar) you can often go back through the chain of citations to find a more accurate and reliable source.

Depending on the subject, searching google books is also helpful.