Anybody knows where to get it? (I have already searched through several websites, please reply only if you are positive :) )
I need it for my SkyRay STL-V2
^ I'm sorry for intervening and neglecting your wish but after waiting for one week I'd like to bump this eventually up.
It seems (based e.g. on this post https://budgetlightforum.com/t/-/5656#comment-107001) that a C8 extension tube will fit and after googling I found easily several possibilities. E.g. this combination looks pretty interesting:
Is anybody able to confirm that it looks like a correct one for SkyRay STL-V2 (or similar)? Of course the quality of the pressure switch is another story... C8 tubes seem to go for ~USD10
Might be although things are relative... I bit the bullet and made an offer for that combo, it was accepted and now I have to pay less than 50USD for this STL-V2 setup (incl. the light and 18650 + 16340 batteries of course). Let's hope it fits in the STL-V2 body
How can you tell it's better...
Maybe one of these?
By comparing to the original tube/tailcap on the middle you will easily notice the tube linked above does not fit at least in Skyray STL-V2 
OT: Now I might have to find a head/1x18650 pill combination that fits in this otherwise useless extension tube although I was originally sort of planning to stick with just STL-V2 in these type of lights...
Edit: I couldn't resist ordering a XM-L C8 from KD...
^ OK, I understand... So you have already a tube that fits STL-V2 - is this the one you were looking for in the OP?
In the best case I'll have an extension w. a pressure switch that fits my KD C8 as also the KD C8's tube that fits my STL-V2 (to make it run on 1x18650) but in the worst case "just" a useless extension but still quite nice combination of two XM-L lights, the 2x18650 one (STL-V2) being more throwy and the 1x18650 shorty (C8, OP reflector) with perhaps quite useful, more floody beam. BTW. STL-V2 is quite compact when running it on 3x16340. Not for trouser pocket carry though but jacket pocket/holster carry is fine.