Recording an Interview

It may be tempting to think that you will remember what you are being told, or to try to jot down notes while the person  is talking.  In a jam, you do what  you have to do, but if you have some time to prepare, it is ideal to record the  interview.  Writing while a person  is talking makes you look distracted, it makes it difficult for the subject to  “discuss” a topic with you, and instead makes them feel like they are being  tested or like you are only interested in getting to that “end goal”of  yours.  You might still be able to  get some valuable information, but you are really missing out if your subject  doesn’t feel comfortable enough to open up to you! The other benefit to a  recording, is the ability to go over it a second (or third) time.   Especially if you are nervous or uncomfortable, but even if you are just trying to remember the next question, it is easy to miss bits and pieces of what is said, or to misinterpret information the first time.   A recording provides an accurate record, something that can be referred to as a source and that can be kept for posterity.  
In-Person  Interviews
There are decent recording devices available at any  store with an electronics department from $20-$70.   Keep in mind that often the less expensive models will lack in voice  quality.  If you are recording this just for yourself you may not be too concerned with the end quality, however, if you think you might like to keep the recording, you might want to make sure that the recorder you buy has the capability to hook up to an external  microphone.  Check out Amazon for a  few examples.  
Long Distance Options  
If you are recording an interview that is not in  person, there are a few different routes you can take.   The simplest method involves ye old smart phone.  If you have a smart phone, it is incredibly easy to record a phone call without spending any money at all.  Check out this tutorial using the app iPadio to record a call.  
Another easy option is Skype.  Using free Skype recording software you can get a pretty decent quality recording for a fairly low price.  
For more information on recording phone calls  check out this comprehensive list by Dan Curtis, a professional Personal Historian (i.e. he interviews people like this for a living).