There is something called Dekstrometorfan that I had access to a few years back in concentrated pink little pills "DXM" that at a very bad point in my life actually helped me immensely that I would know like to know how to get more off. I'm fairly sure what I had isn't available anymore so looking for e.g. cough syrup containing it or anything else would be a suitable substitute.
I was actually surprised by it's effect when I did take it as it "cured" for a lack of a better word a deep depression and anxiety I was experiencing since a long time, this helped suppress that for about 2 months. My doctors has never been able to find anything better and the quality of my life is so poor I'm willing to do a little work myself.
Are there any online sources or local pharmacies where I could get this over-the-counter?
The most popular is Acodin. I think all online Polish pharmacies have it. Search for 'apteka internetowa acodin' in google. Generally you may buy only 2 packs in one order, but in some places you can buy also more at once. Five packs here: