Yeah, buy more yeast. I've done it before on a few Oktoberfests that I didn't have starters for. Nothing wrong with doing what you need to do to meet a brew day and make sure you have a good fermentation.
OR... you could make a starter, brew as usual, and hold the wort until the starter's ready. As long as your sanitization is good, you'll be fine.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein