from breast milk to formula

Hi ladies 
So I am ebf. 
I have pumped and my son takes a bottle just fine. 
I am not producing as much and know I will have to supplement soon. I want to slowly transition but have no idea where or what to buy. 
Formula- what brand is best? Similar to mommas breast milk? Price doesn't matter I want the best. I would say my son is pretty health no tummy problems or allergies.
Feeding- so I read I should mix formula with breast milk to transition? 
Is there a ratio? For how long? 
Thanks for all the help! 😊