Their are many many upholstery shops in the LA area. Check in the yellow pages then go visit them and see the quality of their work. Most will have their own fabric sources and many will take COM, but their labor cost will increase some. But their also many fabric outlets that you can save big on fabrics.
Either take the cushions to them or for a more precise fit they can come and measure.

Ditto, just about any furniture re-upholstery shop can remake cores and covers.

If I remember correctly UFO has several outlets in the San Diego area and usually has their own shop inside their store. Maybe some similar type outlets in LA.

Here is a link that might be useful: ufo

Hello,
You must contact the store from where you purchased these sofa cushions. If they are in warranty then they will surely replace it. For new cushion try online stores where you can find the best one.