You called it a backup list, but isn't that just planning for the next rush order fail? Most local farms can't do last-minute either. You still need to call ahead. I tried to get peonies from a farm same-day last spring and they just laughed. The real move is building a relationship and ordering early, not keeping a list for emergencies.
Actually a backup list isn't for same-day calls, it's for when your main grower's crop fails. Like if their peonies get hit by frost. Building relationships is key, but having a second farm you also work with means you're not stuck. It's not either/or, you do both!