The city of LA has both ground up tree trimmings and compost that they will deliver, but it's a lot. The smallest is a small truck load. You can get less if you're willing to go to the site and get it yourself. It's free!

The phone is 213-923-3109

Check the phone book under "top soil", I think. That's how I found compost/mulch suppliers here in The OC. May be hard to get bark from a local supplier--wood is mostly Eucalyptus around here--local mulch suppliers usually have composted tree/gardeners trimmings. Bark usually comes from lumber mills, where they strip the bark off before the milling process.

My dogs would make a holy mess of bark mulch--sticks to their fur, flies everywhere, comes in the house with them and lodges in the rug. My dog area is St. Augustine grass. I HATE St. Augustine grass, but it holds up to dog activity very well, and it is only in that one area, just for the pups. They like it fine.