Getting a simple PAYG sim really quick?

[Vanilla]

I have a friend in the country for a few weeks who doens't have a phone.

I'd like to arm them with a PAYG chip and one of my old phones.

I know there are always post orders where Orange, O2, etc will send you SIMs but can you walk into any phone shop and grab one of these? Or will they charge for the SIM?

Thanks