Prepaid Sim for data in US?

[Kricke]

Hi,

I live in Germany and want to go to the US in June for about 3 weeks.
I plan to travel with my iPad Wifi only. And I do have a smartphone which I use to setup an access point for the iPad.
I already found a prepaid sim card for my phone to make inexpensive calls home and within the US.

Now I am still looking for a prepaid data sim card for my huawei hotspot.

Where can I buy such a card if one is available?
What are your suggestions?
Or am I totally wrong and there are no such prepaid data sim cards?

Thanks.

Knut