If you really need a German number, you can sign up for prepaid with any of the German networks or MVNOs. Germany requires ID verification for mobile service. Some carriers have a way to submit it online, but a couple MVNOs get around it by allowing only PayPal or or direct debit. You can sign up from the U.S. but it probably won't activate until you're in Germany. This is far easier if you speak German.

If you don't need a German number, Airalo, Vodafone Travel, etc. will be fine. You can get by just fine doing normal tourist things without a local number. WhatsApp is everywhere, so you'll be able to WhatsApp your tour guide, the guy or girl from the bar last night, etc. On the off chance that you actually have to call someone's landline, you can pay the 1-2c/min to do it on Google Voice or the VoIP provider of your choice.