Buy the best you can afford. Pay attention to recording quality and microphone capabilities. Do not purchase one that does not support mp3 if you plan to place these online.
Audio Recorder Brands:
iPhone and iPad:
Reviews for iOS microphones: http://www.micreviews.com/guides/top-10-best-ios-microphones
Phone settings while recording: http://www.oralhistorycentre.ca/introduction-recording-interviews-ios-devices-smartphones-and-tablets
Free software for iPhone and iPad (get through the App Store):
Voice Recorder & Audio Editor (TapMedia)
Voice Record Pro (Dayana)
There are free and paid apps for Android phones, as well - check reviews to help make your selection.
Audacity (free) - http://www.audacityteam.org/
OcenAudio (free) - https://www.ocenaudio.com/
Adobe Audition ($$$) - https://www.adobe.com/products/audition.html
Others rated well ($) - WavePad, Acoustica, Sound Forge