I had front teeth extracted and a bone graft for implant about 7 days ago. I have flipper but it is not comfortable.

Flippers are never the most comfortable thing you can have in your mouth. They are never that stable, strong or capable of providing any form of function (eating). but they should be adjusted so your back teeth do remain in contact during the healing phase.  You should return to your doctor to get an adjustment to ensure your back teeth can come into contact.   
Another option that may be available, and this depends on your situation, is bonding a temporary tooth in the gap. The bond would be to the teeth on either side of the gap so those teeth would have to be healthy enough to bond too ( If there are crowns on the adjacent teeth the bonding won't be an option) 
A bonded temporary tooth can be a nice aesthetic option that is more comfortable than a flipper but can only be used in certain situations and like the flipper can't be used for biting on. It is meant as a temporary solution and has to be treated very carefully while you are waiting on the Implant treatment.