AFIK the majority, if not all, travelcards start at zone 1 and then expand outwards, i.e. you could buy 1-4, or 1-6, or 1-9, rather than purchasing per each individual zone you plan to travel through. I'd also sit down and work out if it actually makes financial sense to buy a travel card; nowadays, if you have a contactless debit card/phone payments, it may be easier and more economical to just use that for payments on tfl (remember there's a daily cap on the tube fare), especially if you're not planning on making lots of journeys to outer zones.