Have you checked a salvage/reuse center? Most older wood doors are solid.

However, "leaky" really doesn't have much to do with whether the door is hollow or not. Leaky means airs is getting in around the perimeter of the door. You can probably fix that with a sweep at the bottom and weather stripping on the sides and top.

That is true and we have done the weatherstripping and door sweep. I just assumed that a hollow door would not be a good option for an outside door. I found a salvage place around here . . . I just have to be willing to sort through stacks and stacks of doors to find the right size. It is not a very organized place!

I didn't mean to imply a hollow interior door was appropriate. I just wanted to make sure you were doing the things that would have the biggest impact. A solid door with a gap at the bottom is going to let out way more heat than a hollow door that is properly sealed. Obviously, the best option is to have an exterior quality door that is properly sealed.

If the weatherstripping is done and the door needs some insulation, consider some flat insulation on the back side where you won't see it.