If you’ve got less than a week to go, I’d strongly recommend using the last minute sit thread: How to use the Last Minute Sit Category - #13 by anon27523372
Otherwise, it’s unfortunate that THS doesn’t keep tabs on sitters or petparents who cancel, so a sitter with a great track record on completed sits, might have a secret tendency to cancel that won’t be recorded. If the reasons for cancelling seem flaky, you can report this to THS. They will “watch” the sitter’s account, but as most cancellations are “undisputed” the patterns can continue under the radar.
The best thing you can do is get in touch with previous sitters if you have any to see if they’re interested. You can also start a forum thread to work on your listing and make it the best it can be to attract sitters.
I tend to look for sitters who I feel comfortable with and who have a record of trust, not just based on their sits, but also external references, and even linkedin and airbnb. The more trustworthy the person is, the less likely to flake on a sit for no good reason.