The Fed has the privilege to buy notes from the Bureau, treating them as physical products that have been manufactured. Last I heard the price of notes was something like 3% of face value.
To expand on Gaz' response, here's a .pdf from the BEP the intro of which contains this quote. I put the purchase with the wrong agency (banking institutions buy it, not the Fed. they just order it from BEP/Mint).
Depository institutions, such as commercial banks, savings banks, savings and loan associations, and credit unions, buy currency from Federal Reserve banks to meet customer demand. The value of that cash is charged to their accounts and when the need for currency and coin declines, these institutions return excess cash to a Federal Reserve bank, which in turn credits the appropriate account.
It sounds like you're talking about different things?
Currency starts out as plain paper, worth a small fraction of the number printed on it. Somewhere along the line, a miracle happens, and it becomes real money. After that, a bank can get some only by giving up an equal amount of its reserves at a Federal Reserve Bank.
I'm trying to figure out exactly where the miracle happens. Who is holding the paper when it turns into money? (And how do I get their job?)