I’ve got several, one cost £9 from a garage, another I got from an ex-Motability car, and my car came with one. I’ve used loads at work, flat tyres, like flat batteries are a fact of life, and often sorted by just grabbing a random 12v inflator from the nearest car with one.
Yes, they’re staggeringly noisy, best to stand twenty feet away while it’s doing it’s little dance by the flat tyre, but I’ve never found one that didn’t do it’s basic job of inflating a flat tyre.
Don’t use the white goo that’s often supplied, the tyre will be trashed if you do – it’s impossible to mend a puncture if that stuff is inside the tyre. They do get a bit warm as well.