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Looks like you want a minijack to minijack cable. I really don't buy into expensive cabling, but in this instance cheap stuff really can be quite crappy so buy a nice one that looks good and doesn't spend its life wanting to tie itself in knots. Anything over £15 could well be snake oil tho'....
Well, they include a cable to connect the speakers to each other, according to the manual. If that's what the OP hasn't got, suggest a call to Ruark is in order.
However, being basically designed as a Bluetooth speaker, with the bonus of a 3.5mm lin-in jack, perhaps it's unfair to expect a 3.5mm jack cable to be included (and as someone has already said, a 3.5mm-to-3.5mm is what's needed).
Bit baffled about why anyone's using an HRT Microstreamer into the MR1s, though, unless it's to play 24/96 tracks or something. Surely Bluetooth from laptop to MR1 is the way to go, or am I missing something?