I haven't rented since I own. But every single time I have picked up permits, I've seen people renting canisters at the Interagency permit center, and have never seen or heard of them running out. I doubt you need to stress on this.
If you really want to plan ahead, you can rent one from REI. But again, I doubt you'll have any problem at the permit center.
By the way, a canister is far larger than you need for one person, one night. A Garcia canister (which is all they rent) can take one person 7 days or so. If there are a few of you, you can share one canister for a Whitney overnight.
