I use the one from Japan Exposures Store (JE) here:
http://www.japanexposures.com/shop/accessories/viewfinder/page-2/
They available in 3 different magnifications (these being 1.15x, 1.35x and 0.85x) but the real advantage for me (and which answers your question) is that each one also has infinitely variable diopter adjustment between -3 and +1.5, This worked well for me given my eyesight.
To be honest this was the real motivation for me to buy it, as I was having serious trouble focusing my M8 with any success. The Leica diopters are not readily available in Australia and to buy one on the internet would mean I would have had the problem of estimating the diopter required which maybe meant sending it back and exchanging it if it was not right. Not to mention the problem that my eyes almost certainly would (and have) changed over time. Once I got it of course I realized that its magnification (mine is 1.35x) is an advantage too as I mainly like to shoot in longer focal lengths and this magnification helps focus accuracy as effectively it is much the same as increasing the baseline length of the rangefinder.
The price is 19,500 yen.
BTW for really critical focusing I sometimes stack this on top of an aftermarket magnifier (which is not diopter corrected) which I bought on eBay. I bought that one first, hoping it would solve my focusing issues before I knew about the JE one. It did not, hence my decision to by the JE one once I became aware of its availability. The other advantage of stacking is that the two together increase the eye relief quite a bit which helps prevent you from having to smoosh your nose against the LCD screen quite so much if you use your left eye, as I do.
I cannot comment on the optical quality of the JE magnifier/ diopter lens except to say it is OK and works (the M8 finder is itself, truth be told, a bit crappy by comparison with earlier film M cameras and the new M10 finder). I have no way to compare it with the Leica magnifier but suspect the latter has better optical quality. But given it has no diopter adjustment that is a moot point for me.
The only slight problem I have noticed is that the JE model is constructed from two separate lens units that screw together to achieve the diopter adjustment - rotating the rear unit changes the diopter. But the threads can be a bit loose and change by themselves if you hang the camera around your neck on a strap - caused by contact with your body when walking about. This also raises a risk that the rear lens unit can unscrew totally and become lost. To avoid this you might put a small dob of fingernail polish on the treads to lock them in place but this means using a bit of force to break the bond if you do need to make changes to the diopter setting. Or I instead used "Bluetack" putty / stickum (I don't know what it is called) rubbed into the threads to tighten them. This is not a complete bond and allows the diopter to be changed easily but seems to stop the above problem.
https://en.wikipedia.org/wiki/Blu-Tack