I sleep, (yeah, right, if you can call it that), in a recliner because of my back, and when I first started to have to wear diapers, I would use a regular mattress pad. After peeing on my chair a few times because the disposable diapers, Depends, leaked, my wife bought me a couple of the normal mattress pads to protect the chair with.
Now that I've switched to using thick cloth diapers and plastic pants, I don't have any leaks anymore, and I've been able to do away with the pads.
But you can make one out of a trash bag if you need one in a hurry. It's cheap, but effective.