In theory, for the most faithful reproduction, you want low-resistance (ie. thick) cables. TBH, much of the (admittedly mainly male) obsession with speaker cable is tosh. Get something decent, but you don't need gold plated, oxygen free, uni-directional, plaitted cables blessed by the Dalai Lama (although doubtless it might make your music sound that teensiest bit better, LOL).
The flat cable does come in different 'equivalent ratings' (thicknesses), but I bet even then cheapest and thinnest won't pose a problem for normal hifi and normal speakers. If you're driving a particularly tricky load, or have a particularly powerful amp, find a specialist one which is a tiny bit heavier / thicker... it'll still do the trick though.
You can find it in the UK, I haven't bought any recently though, sorry.