where can I catch a shuttle from LAX to rental location?

I think your stretching the 15 mins. It's about a 30 min wait for the shuttles, last 3 times I've rented. But the agents make themselves available very quick, that's different from other agencies. Only changes I see that can be improved faster shuttle, and deposits are HIGH compared to other agencies. They do offer a nicer line of cars, but $1k deposit for a Cadillac is high.

They have shuttles every 10 minutes or so

after you get out of the airport, go to the right side, I'll see a laugh with Sixt. there stops the bus that takes you to the location. the bus comes from 15 in 15 minutes. you will see a Sixt bus ... success and fun !!