Is there some places where I can buy very cheap traffic for a adsense site ?

My ctr is 2-4% and I get about 0.40$ pr click.

I didn't think arbitrage was supposed to work anymore, so probably not.

You can try traffic exchanges or safelists. It should bring you huge amount of traffic in very short period of time. Go Google it.

But is it "good traffic" ?

Good traffic always comes in a natural way. Don't expect much in paid traffic. Work hard instead.

dont use it, google knows if you use some method to increase your traffic, use naturally