Permethylated ethylamines are normally obtained from the parent ethylamine, in a mass-production. I've forgotten the details, but it resembles: mix with methanol and formaldehyde, stir and heat, done. The permethylated ethylamines are industrial compounds. In drum amounts they should cost like 3usd/kg.
So wouldn't it be easier to find the permethylated thing ready to buy? Or if not available, to methylate in your lab the ethyleneamine?