can anyone recommend me which kind of chop cup is the best deal to get
and from which supplier?

I suggest you to get it TOGETHER with your set of Cups and Balls, so the cups will not be different.

whats the difference between brass, aluminium and copper cups?penguin sells a set of chop cup at 19.95usd while Murphy sells a set of chop cup at 8usd. I'm wondering if cheaper means lacking in quality and durability.

Not much difference except for the weight and the look. Most people will prefer brass as it is heavier and it sounds better too when they clink against other cups.