You can check this using the "Online FERRY TICKETS to the island" system on this web if you shall take ferry from Kilini. If you take ferry from Lefkada (Ports of Nidri or Vasiliki) or from Ithaca (Port Frikes, Vathy or Pisaetos) this is not posiable, because ferry companies are not in reservation systems. You can find ferry companies phones in Internet using serch with names of ports. If you caming from Zakynthos (Port Agios Nikalos) you can visit web
http://www.zanteweb.gr and find phone nomber in section Transportaitin. The tickets have normal prices and can say that they are chip.