Hell, they have them at the supermarket checkout.
Check whether your mobile provider has an add-on or bolt-on or whatever-on for your plan to give you international doin's. Might be cheaper than the alternative.
Edit: Make sure you get the right size!
Linkypoos:
http://www.telegraph.co.uk/travel/travel-advice/9432416/The-best-local-SIM-cards-in-Europe.html
http://www.tripadvisor.co.uk/ShowTopic-g34515-i19-k5360886-US_SIM_for_a_UK_phone-Orlando_Florida.html
https://www.bing.com/search?q=can+i+buy+a+sim+in+europe&PC=U316&FORM=CHROMN&adlt=strict
https://www.bing.com/search?q=can+americans+buy+uk+sim+cards+on+holiday&PC=U316&FORM=CHROMN&adlt=strict
Check whether your mobile provider has an add-on or bolt-on or whatever-on for your plan to give you international doin's. Might be cheaper than the alternative.
Edit: Make sure you get the right size!
Linkypoos:
http://www.telegraph.co.uk/travel/travel-advice/9432416/The-best-local-SIM-cards-in-Europe.html
http://www.tripadvisor.co.uk/ShowTopic-g34515-i19-k5360886-US_SIM_for_a_UK_phone-Orlando_Florida.html
https://www.bing.com/search?q=can+i+buy+a+sim+in+europe&PC=U316&FORM=CHROMN&adlt=strict
https://www.bing.com/search?q=can+americans+buy+uk+sim+cards+on+holiday&PC=U316&FORM=CHROMN&adlt=strict