Currently i'm working in Paper Products company manager.How to convince customer to buy mine if the reply that we've contract with other supplier?

When I was in Procurement field prior shifting to IT, the most effective sales people I've seen are those who can demonstrate that their product is better than the competition and provide a good deal that is hard to reject.

Example: If you are offering paper e.g. rim of bond paper as a sample why not offer something like on the first6 months to get from you and give them1 rim free. Better if you can find an inferior product then do a demo like ink test or wet test(paper can easily torn off) then after the demo give the company1 rim and evaluate it... I mean something like that.

I've been buying a lot in China and South East Asia before so I've seen most tactics just to sell their products...

Offer free samples. demonstrate how your product is better than others and provide competitive pricing.