Firstly - thanks for the look!
On your question... Have gotten that a few times... here's an ad firm that sells "retail" [0] and has rates or you can go directly to Outfront [1] if you want buy in bulk (unit pricing will be multiples cheaper).
[0] https://www.bluelinemedia.com/subway-advertising#rates [1] https://www.outfront.com/mta
But that leads me to a question I've really wanted to ask... how much does it cost to put up an ad on the subway? (if you don't mind answering)