Look at

"Self-associated, “Distillable” Ionic Media"

http://mdpi.org/molecules/papers/90600387.pdf  - and citation therein

Thank you @ Andrei Rogoza

Synthesize N,N-dimethylammonium N’,N’-dimethylcarbamate: React dimethyl hydroxyl amine with N,N-dimethyl isocyanate  in benzene under reflux  to get desired product.

I would say the easiest way is to add dry ice to a dimethylamine solution in an organic non polar and aprotic  solvent.