Check out this great DIY Sisal Scratching Post project for those who want to refurbish an old (well-loved) sisal scratching board (via Practically Functional). Or, for you more adventurous handyfolk, take the information there and use it to create something original that your cat will love. I have big male cats and have often found that the scratching boards are just too short for them to get a really full stretch while scratching, so I might try getting a much longer plank of wood and covering it in the sisal rope. 

Whatever you decide to create, your cats (and your budget) will thank you for it!