The CRSP Treasury set does not carry a series that corresponds to a 30-day TBill return at the daily frequency. They do have monthly "risk-free" interest rates, however, and commercial 30-days CD rates at the daily frequency.
See CRSP Calendar File in their Daily Treasury datasets (which also carry daily quotations for individual bonds).
For more on daily TBill rates, see Federal Reserve Bank - Interest Rates Data
This set is collected from the FRB H15 report ( www.federalreserve.gov/releases/h15/data.htm )