Are you asking for a particular product you can buy or the type of microphone technology that works best?
As the Korotkoff is within the audio range, I would think almost any type of microphone would work, excluding exterior sounds is probably it's most critical parameter.
Many "dynamic microphones" (cone, coil and magnet like a small speaker) cut low frequencies and might not be good for the low frequencies you want. Use an electret microphone.
To hear the heartbeats on the chest I mounted an electret mic on the plastic lid from a small jar of peanut butter. The lid blocks background noises.