If you are going to Sitka I recommend the bear rescue center. Sighting bears is "no guarantee"
However - also in Juneau at Mendenhall Glacier there is a bear viewing area and I have had luck both times I have been there.
Just be advised that bears do not keep a strict schedule, so you never know. I was on a week-long trip on a small excursion boat and we never saw any bears - plenty of whales, though.