The pound sterling is the currency of the United Kingdom, which consists of England, Scotland, Wales and Northern Ireland. There are various ways to buy it with U.S. currency, although it may be difficult to buy only one pound sterling at a time, as most methods of exchanging currency require a minimum purchase.
Most banks in the U.S. hold some amount of various foreign currencies on-site. Simply walk up to a teller and ask for a pound sterling, or the minimum amount required by the bank. The teller will charge you the equivalent in U.S. currency.
Currency Exchange Booths
You can find currency exchange booths in most major U.S. airports and in the centers of large U.S. cities. Ask for a pound sterling, or the minimum required amount. You will be charged the equivalent in U.S. currency.
Banking Machines in the U.K.
You can also use your bank card or credit card in an ATM anywhere in the United Kingdom. You will receive pounds sterling, and technically you will be using U.S. currency, since your bank or credit card issuer will convert the withdrawal and debit your account in U.S. currency.