Don't confuse DXM (dextromethorphan) with DPH (diphenhydramine, sominex etc)! The first is a dissociative/NMDA antagonist while the second, DPH, is a delirant - nothing you want to ingest, specially not while withdrawing. DPH can be a good sleep aid for those who respond to it but take just one, max two pills!
Memantine can be bought from Indian generics online pharmacies. Depends on your country if you're allowed to buy medicine from abroad, check the law first. DXM is over-the-counter in most countries including the UK. It's the ingredient in Wick Formula 55 and some Robitussin brands but if possible you want to get gelcaps/capsules not syrup to avoid nausea from downing large amounts of syrup.
Possibly you don't even need such big amounts, but probably at least 150-250mg. Some people react to less, for me threshold was with 75mg. This will induce a slight dissociative trip (read up in the DXM FAQ by W. White if you're interested) and hopefully reset part of your tolerance.
Yeah, PAWS is evil. But in my case I found memantine to aid with this as well. I think every dissociative should do because all of them are dopaminergic but memantine is additionally a direct dopamine D2 agonist (basically the inverse effect of antipsychotics which cause anhedonia, akathisia/restlessness etc) which might make it better in treating PAWS than other dissos. Also memantine is almost free of a trip.