Well, mirtazapine isn't scheduled in the US so you can probably get some precursors with relative ease. You can find the synthesis info online [1] and at least some of the chemicals used in the synthesis are available online [2], though for the price it seems like it might be more economically feasible to just buy a pill from a pharma company.
Also, I found ASX-05 (Phase III) uses a DM metabolite that is retained much more by the addition of brupropion. DM is the stuff in some cough syrups.