where do you report someone for getting boosted ?

Submit a support ticket https://support.riotgames.com/hc/en-us/requests/new

you can always submit a ticket, but how do you know theyre boosted though?

you can do a post game report or support ticket if you suspect boosting but you'll have to most likely do a pair of reports since you can't put two people in one report (booster and boosted)