We have owned the Calpol thermometer for about 3 years and have found it to be excellent, we have been through several packs of probe covers and never had a problem finding them until recently.
Calpol no longer market the thermometer or probe covers. However I telephoned their helpline and was told that the covers are still manufactured and available from another source but not under the Calpol brand. I don't want to risk this review being removed by Amazon because I directly mention another website so I'm afraid you'll have to do a bit of digging, although I'll try to give you as much help as I can. Search (using your favourite search engine) for "Microlife Digital Ear Thermometer Probe Covers" and if the result contains the word "Dolby", then bingo you have found them. I hope that helps.
The thermometer itself is sold on Amazon by Dolby Healthcare here: Microlife IR 1DE1 Infrared Ear Thermometer
although the replacement probe covers don't appear to be (yet), hopefully someone will see sense and add them to the products available through Amazon.