I've never understood how the springs are rated or what exactly the numbers relate to. Most people agree that the 1500 or 2000 mean rpm over OEM opening but in reality it is torque that forces the pulley set open.
I have an old bathroom scale that I use to push the spring till the coils almost touch. That is how I know which is stiffer. I tried a Stage 6, three spring set, and they were all weaker than the Malossi. Same for the Polini Evo two spring set.
If Nakuru/MHR make one comp. to the Malossi I do not know.
You can get the Malossi anywhere. It's the same size spring that fits in almost every 50cc scooter built. 2t or 4t, it makes no difference.
I'd go thru Louie at SPI or racing Planet. They are the only two shops I deal with. (even tho I hate RP's inventory policies). Scooter west or Treatland are good choices too. Bengi from Treatland is one of the best people you will run across.