Wanna Make A Lego MOUNTAIN! (Wanna buy Kilos of Grey Lego...)

Have you tried bricklink?

You might want to have a browse of the mountainy base plates that have been released over the years such as this one:

bricklink.com/v2/…px2

One of those - if appropriate - will allow you to build the first Y cm’s of your mountains without having to use loads of unnecessary bricks and give you a solid base.

Good luck (edited)