I used ASA2fly.com for all of my exams and was quite happy with it. Gleimaviation.com also has some decent stuff, as well as Sheppard. Don't know about Aviationexam and Dauntless regarding FAA testing.
I am going to take my written exams for the FAA PPL, IR and CPL soon.
The question is, will you be doing your training according to Part 141 or 61? Part 61 provides "home study" courses which can be done in advance (or independently) form the training syllabus, basically you could/can take your written test/s before you start with flight training. Under Part 141 this is (usually) not possible, would require the ok of the flight school, because the training (ground school, testing etc.) must be in accordance with the syllabus. In both cases (Part 141 or 61), prior taking any test an instructor endorsement is required.