The first hurdle is finding the monologue. It needs to fit your actor type and any requirements of the audition posting.
While I don’t recommend this as your regular course of action, this is a good time to pull out the monologue compilation books, with one caveat: make sure the monologue you are pulling is from a published play. Some monologue books are collections of monologues someone has written to publish as a selection of monologues; these are rarely appropriate for an audition setting.
How can you know? Is the book “written by” or “edited by?” Does the table of contents list the play and playwright from which the monologue is pulled? If it is edited by someone and includes the play and playwright the monologue is selected from, then it is a collection of monologues from published plays. If it is written by someone, it is a collection of monologues that are not written within the context of a larger story and are not appropriate to use in an audition setting.
It is preferable to read the whole play, but reading a substantial summary will do in an extreme pinch. Please note that you are unlikely to be successful in auditions for summer intensives and college auditions with this method, but it will do for some of your auditions for local productions.
Another good source of monologues from published plays are ten minute play collections. It is easy to flip through the plays and see which one might have a character that is a good fit for you. Then, since it is a short play, you will be able to read the whole play within 15 minutes. This is a solution I am more comfortable recommending, but it requires that you have access to collections of ten minute plays. My favorite place to find these are used book stores.
A monologue selected at the last minute is never going to be as emotionally resonant as one you’ve had time to live with for awhile, but last minute monologues happen. Give yourself a little grace for whatever the reason is that you’ve found yourself in this situation, and move forward with confidence now that you have a strategy.