Contact the department in which you are writing your thesis. Typically the department will assign each person a thesis adviser. If they didn't, request one. A professor at your school should be your first line of editing, because he/she will know what the department wants. Getting outside editing help does no good if you don't know what your school expects.
Contact academic experts in the field about which you're writing. Scholars tend to love others taking an interest in their topics and will often be happy to help. Obviously asking a stranger to edit a 100+ page document is a little forward, so send the scholar a particularly relevant chapter. Email first to establish contact before you send anything.
Send the thesis to a thesis-editing service. There are several linked in Resources below. For a nominal fee, a professional can look over your grammar, spelling and the strength of your arguments. Note that this should be a final step; the person editing your thesis likely knows little about your field, so you'll want the content and research complete before you send it to the service. They can help fine-tune the language and syntax.
Give the thesis to a friend or family member to look over. An extra pair of eyes can help catch that last typo. To go through it really thoroughly, ask them to read it backwards. While tedious, this trick makes repeated or misspelled words easy to spot.