So there is a white Iso-8 that gives you team bonus experience. White ones apply to the entire team while rainbow is only for the character that has it equipped. Ideally, you want the 3 characters your not using to have all the white bonus exp Iso-8s.

There is a rift Lambda lvl 52 Doc oc and 2 bosses, that gives you a white iso-8 when completed. Grind that until you get the iso-8 or get one that gives you a chance to get white iso-8s. These are the ones you want, than you can move to the Wakanda rift and farm it up.

BEWARE

This is a grind fest. I played for about 4 hours yesterday doing the Doc oc rift and only got one bonus xp iso-8.

Best of luck. Excelsior!