There are several concerns besides "cheapest". An ideal EU travel SIM must satisfy the following properties:
Available for purchase on Ebay or some other website that ships internationally
Can be activated from abroad or can be purchased pre-activated
Its possible to top up the SIM with a foreign credit card without huge surcharges. I.e. Czech ...
While AirBNB may not be as good as a site like that suggested, it does allow filtering the search in many ways. With the criteria shown here, 53 places in the entire country appeared. Narrow done the location and increase the price and see what happens.
I have had this problem as well. If you select 'shopping basket' from the menu in your oyster account, you will find the option to add new purchases, where you can choose auto top up and make the new request.