I'm filing a nonresident NY State return and need to allocate my income based on days worked in NY state and elsewhere. How do i access form IT-203-B in Turbo Tax?

Since you are using TurboTax Online, you don't have direct access to the Forms.  The Download or CD desktop versions have that capability.

TurboTax Online prepares the forms based on your entries to the interview questions.  

Prepare your Non-Resident New York return first, then your resident return.

Click the link for more info on filing Non-Resident states:

https://ttlc.intuit.com/replies/4777207