As you already know, we have to migrate the $BBS to $RTB using metamask... If you don't know how and where to do it, I'll give you a link to a post I published yesterday where I discuss the steps to follow. 

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Now, my question is.... What is the cheapest way to fund a Metamask wallet with ETH? ... I need it to be able to migrate my $BBS and I want to know what is the most convenient way to do it