Help to build Rivendell extended

As I mentioned I need help with the Rivendell extended build by Fanpeixi. I’m currently trying to buy pieces for the extension and the mountainous base, on top of the 2 Rivendell sets I purchased. However, I do know you can replace the 2x6, 2x8 and 2x10 bricks and plates by any color, but is there any other piece you could do this with such as the 1x1 bricks or others? If not, any possibility to replace a certain quantity of them?

Also, does anybody know the cheapest website to buy bulk off brand Lego?

(Sorry if my English is bad, not my first language)