Most people use small hardware store nylon or aluminum washers, 3mm inside diameter and various OD.

I've read of others using credit cards and cutting mats, any thin durable plastic, by drilling a 3mm hole and cutting out washers.

The washers I've been using with great success are nylon and made for another 450 heli. They fit inside the span of the grip but I think bigger than the grip works OK too.

To keep things balanced use one washer on the top and bottom of each blade.