Depends where you're going I guess, but mobile phone shops are everywhere in most cities these days. Turns out the locals use them too!
You can also get them at the airport a lot of the time, sometimes even from vending machine.
Aye, I was just thinking back to a recent holiday in the UK which was punctuated with a 3-day work-related hop to Belgium.
Arrived at Gatwick with the jetlag & general PO’d-with-the-worldness I’ve always had after long travels, and I’d read that the SIMs available at Gatwick were really exxy. Picked up a car & drove off into the never-never, and the next day we had to take a trip to a (huge) Tesco in order to get a SIM. Really not a problem, we were there a month, and wanted to buy some other stuff anyway (I always forget to take my sunnies!).
However; said work trip to Ghent involved arriving at BRU at 7:30pm, renting a car (traffic didn’t exist - BE World Cup game was on
), arriving at AirBnB accomodation after 9, then finding a tram-ticket vending machine & some dinner ... nothing open at that time of night, at least there, seems to have affordable SIMs. The next 3 days were pretty long & involved getting to/from the office by tram, and it would’ve been a PITA to have to track-down a SIM.
So while I’m not saying it’s super difficult, in a situation like that I’d have swallowed a $5/day or even a $10/day fee to avoid hunting a SIM down. At least, for that short period.