T-Mobile has increased their ownership of low band spectrum licenses to cover over 200 million POPs (population).  The map above depicts the 700MHz A-Block spectrum owner for each US county.  From the map you can see that significant portions of the 700 MHz A-Block spectrum are controlled by parties that are not likely to sell, like US Cellular and C Spire.  Cavalier and Charter are signficant (land mass) spectrum holders that would be likely sellers to T-Mobile.

Below you can see the population counts for each of the owners.  After T-Mobile's 201 million, Cavalier covers 26 million POPs, US Cellular 35 million POPs, C Spire 14 million POPs, Continuum 14 million POPs, and Laser has 10 million POPs in Chicago.