Just to emphasise. You can't look at awards on Air Canada (priced in THEIR miles/points ) and pay with United miles. (Think of them as different currencies!) You CAN, as A Little Cow said, use United miles to purchase AC flights... but that must be through United. I do not know if United has a part miles/part cash payment option but if you don't have enough miles alone to redeem flights that might be worth checking. OR perhaps (assuming it is TWO seats you want) look at ONE award and one paid?