If I want to launch a crypto wallet, how can I secure a sponsoring bank for a crypto money transfer license?

The answer to this question is if you want to start a crypto wallet in the United States that will handle just crypto (and only crypto), you don't need a money transmitter license; you don't need a bank to sponsor you. But the caveat is so far as in today; you don't need a money transmitter license. Will that change tomorrow? Possibly can most likely well, but as of today, you don't need a money transmitter license just for a crypto wallet.

As long as you don't have custody of the private key, that means you're not taking custody of funds (you cannot take custody of funds, even crypto funds), and they are with the user themselves, you're all fine. You don't need a bank to sponsor you. You can go ahead and start it right now. However, what is essential is to have a compliance program in place and a KYC or KYB program in place. Although it might seem overkill, it's not too bad to think about these things.

I have to be very clear here, IANAL. The laws and regulations are ever-changing in the US. Please consult a licensed attorney in your geographic area who understands this vertical.