Pilot Precise V5 RT refill alternatives
It isn't clear what your objective is, whether you're trying to achieve a different writing result, or it's just that you've only found the complete pens for sale and want a way to reuse the body.

Forget trying to refill rollerball cartridges.  They are a complicated system designed to work together.  The ink needs to be just the right viscosity and other characteristics to be properly dispensed at a certain rate, and then dry quickly without spreading (and it's different from other kinds of inks).  Also, the rollerball tip wears with use.  If it isn't designed with the right long-lasting materials to be refillable, it will (hopefully) last just long enough to ensure you can use up the ink in the cartridge before it starts blobbing or leaking.

The cartridge is similar in size and shape to some other ones.  The writing experience and color characteristics will be different for different brands.  If you are looking for that difference, you can probably find some other refills that will fit.  However, Pilot does sell refills.  The listing on Pilot's web site gives the refill number.  Pilot makes nine ink colors and two point sizes for that pen. They should be available from online retailers and office supply stores.  I went on Amazon and searched "pilot precise v5 refills", and there was a good selection.  

Note, though, that in small quantities, you will pay almost as much for the refills as buying the whole pens in a 12-pack.  Buying refills by the dozen will save you about half the cost of the whole pens.