I purchased an audiobook that I was able to listen to on my iPhone 4S but I bought the 5 and I can't find it. Any ideas

Audiobooks are currently a one-time only download. If it's not on your computer's iTunes (it will be in the Books part of your library if it is) nor on a backup then you will need to copy it from your 4S to your computer's iTunes via File > Transfer Purchases (File > Devices > Transfer Purchases on iTunes 11) and you can then sync it to your iPhone 5