Two of my birds already have rings, my female Indian Ringneck has a numbered metal ring and my newest male Scarlet Chested Parrot has a blue plastic band. I have not been able to locate a supplier of metal rings that doesn't require a minimum order of a size and I need three different sizes. I would probably spend $100 on rings and have a lot of left overs. So plastic bands it is for now.
I will probably do all my birds in stages to avoid upsetting them two much.
I have managed to band all my birds, however three of the bird were able to remove the bands soon after I put them on. The Green Cheeked Conure's being the worst as they both removed there bands quickly and they were one of the main species that I wanted to band to allow me to easily determine parent from offspring.
Most of the birds chewed at the bands until they got used to them.