I would definitely check out: http://www.hotelcasamariposa.net/chirripo.html
for accurate information it seems it is the only trusted site you can see on the web. It is incredible. All of these false information sites like chirripo.com are just charging ridiculous amounts of money to hike up Chirripo. I recommend to search up: https://www.costaricarios.com/adventure-travel-in-costa-rica-chirripo.html
I went a couple of weeks ago and it is the easiest way to get reservations.
Came into isidro last and tried calling the park office 10 times this morning. Could not get ahold of them. So i left my friend in isidro to make the payment at the bank (1.5 hour wait in line). And hopped a bus to the park. I was able to make reservations in person to climb the mountain the next day. And she is currently making payment. So far so good but what a hassle.