There are no walk-in permits anymore. Everything is online through Recreation.gov. Here's the link
I just visited the site, and by setting up the entry point (Overnight or Day-Use), and number of hikers, and then clicking between the overnight and day-use permits, a permit for 2 under day-use just showed up for Friday. (Now it's gone, but one for next Tue showed up.) If you check that web page frequently, you just might get lucky.
There are lots and lots of people who won lottery permits whose plans change, or members drop from their group. When they go to finally pick up their permits, their unused slots show up online (at a random time in the next 24 hours). So you have to keep checking.