How can I replace Kenra Violet Booster ? (want to go silver)

[gras]

Hi
I'm a newbie in the hair coloration world. I've tried to learn the basics of the hair's coloration techniques and the different notations (levels and tones). I would like to make a big change and going silver by myself (not an easy deal).

So to achieve this look I would like to use the same formulation used by Guy Tang () :

Kenra Color Silver Metallic Permanent 7SM at the base with violet booster and Kenra Color Permanent 8SM on mid shaft to ends with violet booster both with 10% Kenra Developer.

But my problem is that where I leave I have not access to that Kenra Violet Booster who is necessary to obtain this metallic look which I want.

Do you know how could I replace this additive ? An other brand or with a simple silver toner ?
Thank you !