Some rabbis insist that Rambam writes that he will contradict himself on several occasions in the Guide. If so, where exactly in the introduction to the Guide does Maimonides say that he will purposely contradict himself? I'm looking for the source?
In the Introductory Remarks to Guide for the Perplexed Maimonides lists seven types of contradictions. The fifth and seventh ones are the what you allude to:
Seventh cause: It is sometimes necessary to introduce such metaphysical matter as may partly be disclosed, but must partly be concealed: while, therefore, on one occasion the object which the author has in view may demand that the metaphysical problem be treated as solved in one way, it may be convenient on another occasion to treat it as solved in the opposite way. The author must endeavour, by concealing the fact as much as possible, to prevent the uneducated reader from perceiving the contradiction.
The fifth cause is traceable to the use of a certain method adopted in teaching and expounding profound problems. Namely, a difficult and obscure theorem must sometimes be mentioned and assumed as known, for the illustration of some elementary and intelligible subject which must be taught beforehand the commencement being always made with the easier thing. The teacher must therefore facilitate, in any manner which he can devise, the explanation of those theorems, which have to be assumed as known, and he must content himself with giving a general though somewhat inaccurate notion on the subject. It is, for the present, explained according to the capacity of the students, that they may comprehend it as far as they are required to understand the subject. Later on, the same subject is thoroughly treated and fully developed in its right place.
Shortly thereafter he states:
Any inconsistency discovered in the present work will be found to arise in consequence of the fifth cause or the seventh. Notice this, consider its truth, and remember it well, lest you misunderstand some of the chapters in this book.
Thus, the fifth and seventh forms involve deliberately being inaccurate in certain instances, and Maimonides confirms that all contradictions in his book are of those forms.