Small, professional, broadcast quality audio recorders have advanced remarkably in the past few years. Today's versions are cheaper, of higher quality, and are easier to use than ever before.
(If you only want an inexpensive voice recorder, skip this post.)
B and H Photo has an excellent primer on what to look for in a portable digital recorders, and how they differ from normal "voice recorders" here:
A few models worth considering:
I recommend buying from B and H Photo because of their excellent customer service and 7 day return policy.
When the recorder arrives, spend a few hours recording test interviews. If the recorder controls are not intuitive and easy to use, return it to B and H and try another model.