I never found an European wide SIM card (without paying roaming of course). If you find one, please let me know.
In Portugal you can easily buy a SIM card. For example, for the company UZO it costs 5 euros and comes with 15 euros of credit!
Please have in mind that if you come from American continent your mobile phone may use a different kind of technology and sometimes it may be incompatible with the European standards.
Wish you a nice WWOOFing,