For your first diagnostic exam, I’d suggest taking the AAMC Official Guide Questions as a half-length diagnostic. Using the score converter in our Create-your-own Study Plan Course, you can convert your percent correct on each section into a predicted scaled score. This gives students a very accurate idea as to where they are starting from.
You can buy the AAMC Official Guide Questions for $10 as a stand-alone product here.
If you want to save money in the long run, however, I would suggest buying the complete AAMC online practice bundle and the AAMC flashcards. This will give you access to every single AAMC practice question you need including the AAMC Official Guide Questions.