For Germany I'd recommend Fonic, since their national and international rates are very cheap (€ 0.09/min to Germany and the US) and their SIM is quite widely available. For details check
Germany - Fonic - Prepaid Wireless Internet Access
Roaming with a German SIM in France doesn't really make sense. You'd have to pay € 0.17/min for inbound calls and outbound calls would be more expensive than with a French SIM. In
this posting I listed a couple of French MVNOs with cheap national and international rates. It seems like
Mobiho (formerly "Primus Mobile") is the only one selling their SIMs offline (
retailer search). Interestingly Mobiho has cheaper international rates than national ones.