after the opening email, some customers replied me they have good suppliers already, what can I do? and how to reply them?

try to show your own advantages, I guess the price can be the first break point,but not the most important if your customers are from Europ

Since they have a good suppliers already, the only thing you can do just send him greeting on holidays and updated products information from time to time, but not too often. I believe your customer would think of you when they need a new products.

the suggestion from the 3rd floor is really good.

Good supplier can be they are offering better services then you do. it's depends.