Josh, when you can't find an answer, be sure to check the Orientation Notes (see the link on the left below the webcam pictures) But it is just fine to ask where to find stuff regardless.
For the canister, I just checked the Orientation notes and found this: (here's a link
Bear Canisters/Other Equip.
Day hikers are not required to carry bear canisters as long as they always keep their food with them. Canisters are required for overnight trips and can be rented at either the Visitors Center or the Whitney Portal Store. If you need other hiking gear, the Whitney Portal Store sells some last minute items, and there are four sporting goods stores in downtown Lone Pine. Also check out stove fuel and alcohol for discussions on buying fuel. Elevation and Lone Pine Sporting Goods also have a limited supply of mountaineering gear available for rental.
I hope others will suggest the Portal walk-in camp sites. If that doesn't work, spend a night at Horseshoe Meadows.