there are many hostels in la paz and climbers don't all assemble to just one.
you can usually strike a conversation with the trip organizers in the center of town and they will tell you of groups or individuals and where you can find them also the bars are good place like "Oliver’s Travels".
as far as hostel goes the “loki” is a little on the high end cost wise but well worth it, its on a quiet street close to the center great big beds good shower and an awesome Irish pub well worth .
sometimes you might bump in to so climbers there.
also i dident stay there but it looked nice and i found some climbing buddys at this place the Wild Rover Backpackers Hostel its a 5min walk from loki so make reservations for bothe places and check both of them out before you check in.