Old style baby monitor - audio only (Wrexham LL11)

We're looking for an old style baby monitor - one of those walkie-talkie style things where you literally get the audio only. We don't really need one that's got a camera on it as it's for monitoring an incubator - the incubator has alarms on it but we can't hear them with the door shut, and can't leave the door open as we need to try to keep the temperature regulated. If it's portable then that's just a bonus.