Just pick a random cellular provider, visit their website, and that’s it? Do they have the cheap pay as you go plans available? Honestly don’t know - we’ve always gone to the local Tesco and asked the sales clerk for a sim on whatever service works best in whatever town we’re in.
For example, if you were coming to visit me in Australia, you could get an Optus eSim for the days you are here: https://www.optus.com.au/prepaid/esim
Annoyingly, its not yet perfect. Vodafone for example can do prepaid, but requires the prepaid physical SIM to be activated first then transferred: a Vodafone store can do it of course, but that's a pain.
