You can buy them directly from the nats website. Go to the postseason tickets page, parking is last tab on right. Limited availability, but spots are still available. I was disappointed you can only buy 1 pass at a time. I had to buy one for each game and had to pay the 4 dollar service fee 3 times.
Many thanks...the ticket office told me I had to have bought parking with the tickets. Crappy lots left - I got U - but better than the Metro.
I'll know better for the NLDS and WS.