I don't know how well it compares to Mestinon, but two different people have now posted about substituting it when you don't have or can't get Mestinon.
I order from iHerb a lot, and the companies listed are all reputable and I think, either on this site or the peripheral neuropathy site, it's been mentioned that both ship overseas.

I don't have MG myself, but my husband does. I read this website to gain info for him--it bothers his eyes to sit and try to read the computer. (My problem is peripheral neuropathy.)