Aaaaghr SWIM just ordered the damn stuff, dimethylsulfate is on it's way.
At least that's what SWIM told me last week.
And well he'll just use the synthese in rhodium then since for methyliodide the synth would need to be adjusted since it is a stronger methylating agent.
Soon SWIM will have enough dimethylsulfate for all his life (2,5 liters hehe).