All you need is hydrochloric acid, arsenic and diphenylamine.https://www.prepchem.com/synthesis-of-adamsite/
It's surprisingly easy to obtain those two items. diphenylamine is used as a treatment to prevent apple rot, banned in Europe but still available in North America from two companies. I checked a bunch of SDS and it's just DPA mixed with a solvent and stabilizer.
Decco U.S. Post-Harvest Inc.
No Scald DPA EC-283
Decco No Scald DPA Aerosol
Pace International, LLC
Shield DPA 15% Super Refined
Pace International LLC ecoFOG-100
Pace International LLC ecoFOG-170 DPA
DPA Super Refined Diphenylamine
Aresnic is a bit more tricky and I don't really know of any ways to easily get arsenious oxide. I did however find quite a few arsenic ores and minerals for sale on ebay. Making the oxide is easy you just have to combust arsenic.