If you are in the UK, the cheapest option seems to be Asda pharmacy.
They do not keep them in stock. You have to preorder and they usually get them in within a couple of days.
When comparing costs, remember you do not need to pay VAT. Your diabetes team should be able to provide a VAT exception certificate.
I also recommend only buying one first time.
Some people get skin reactions or find the Libre is very inaccurate for them so don’t bother beyond their first sensor.
Bear in mind, Libre is not considered to be a CGM.
Typically, CGMs will alert when your BG is out of range or changing quickly. Libre will not do this. However, some people have pimped their Libre with Bluetooth widgets like the Miao Miao.