Split it into two. Refrigerate them. Do another starter in a week or whatever depending on your scheduling with one half. If the starter takes off, great. Refrigerate it, you know you have live yeast that should last a few weeks to a month and a half. If you need more time and can't get any more for sure, leapfrog with the other half. Be OCD with clean and sterile.
Consider this... People heist yeast from bottles that have been sitting on hot store shelves for a year or two. In many cases they find a few live cells they are able to wake up and grow into billions. Your starter should be fine if kept cold and clean.