Alternatively, unless you need 6V somewhere else (or are in Canada ;-), 5V is quite sufficient for the M100 and straight-through cables are cheap; trivial to reverse the polarity: e.g: eBay# 231419524659 $1.62
If you need 6V (or more) there are boost converters, although you'll have to supply the cable and I haven't seen any nicely enclosed like the MyVolts unit: e,g,: eBay# 271306601322 $4.69 M100 running off a 5V USB battery pack:
