I recommend two microphones that can be used for location interviews + skype calls on your end (if used with the above recorders)
Budget option - Shure SM58 Microphone $165
This microphone is used by singers, comedians, presentation speakers and now podcasters. This is a high quality, good microphone that will produce nice results.
You will just want to ensure your guest is talking directly into this microphone as audio will be affected if they talk slightly off mic.
Pro option - Sennheiser MD46 $270
This is the microphone I personally use for recordings and one that is trusted by reporters, radio stations and TV stations around the world.
The big advantage with this microphone is that if your guest starts to talk off mic, this microphone will still capture what’s said.
You’ll need to also buy a pop filter with this microphone which should be between $5-$15.