Quote from IB Salvatore:
You can purchase T-Bills at IB. Send your instructions through our Inquiry Ticket system. Please be very specific about the face value and maturity. Providing a cusip or isin is best.
We do not provide quotes. We execute your order at the best price available. Once you send your ticket, you cannot cancel your order.
There is a 1 million dollar face value minimum, and the margin requirement is 3%.