For data i believe the "3 feel at home" from the UK is the best value, 12GB for 21GBP, no roaming fee, many countries included, remaining data could also be used in the US

Phone simcards i am not sure, but i believe the roaming fees are limited by the EU, from June2017 looks like there wont be any roaming charges any longer
So u could buy any prepaid/pay as you go sim card