Get a taxi to Halfords, buy a battery, fit the battery? it’s only 2 bolts depending on what car you have.
On many modern cars this is a nightmare job though
I’m sure there are still easier cars but while i wouldn’t go as far as to say it’s a nightmare on an E91, you’ve several screws and bolts to locate and remove to get the side panel off, and that’s after you’ve emptied the boot and taken the floor panel out.
Then you’ve a couple of bolts and an awkward and recalcitrant +ve connector pack to unbolt and spring free. You’re then faced with manoeuvring a pretty heavy battery out of its well, pretty much at arms length and without being able to get above it to facilitate lifting.
Then you do the whole thing in reverse to fit the new battery. And there’s the ECU reprogram if you can be arsed to pay BMW to do it.
So not quite a nightmare but bloody awkward and not something I’d expect my 87 year old mum to do, for example. So like the OP, I’d be looking for a mobile fitter to do the job. Or if I was at Halfords anyway, get them to fit it.