The same place it was coming from before this deal. Also, as you mention, $8B is going to fix Sprint's messy balance sheet, which makes future debt discussions much more tenable.
said by Forbes.com :
The $8 billion cash infusion provides Sprint with a significant cushion as it undertakes its expensive 4G LTE network rollout and network modernization project. It will also reduce the companys net debt (long-term debt, excluding operating leases, minus cash) to $6.5 billion from $14.5 billion. As a result, its net leverage the ratio of net debt to EBITDA would decline from about 3x to 1.3x. This could lead to credit rating upgrades, and as a result lower borrowing costs for the company.