Only place I know of would be Bicycles of Ojai, in (you guessed it) Ojai.
Robert Coble is the owner, he did a lot of high-end steel stuff back in the day and now does some repair on the side. Not sure if he can do Ti (doubt it) but could probably do a stem repair. He can take a long time to get a project done, but he is a true genius in the literal sense and if you have a special stem you want to modify for long-term use he would be able to help you out I would think...although not sure why you would go through the hassle considering the time put in to modify a stem and repaint if necessary versus just buying a new one.
Anyways, good luck!
BTW for full disclosure I used to work at Bicycles of Ojai, but that really doesn't change my opinion too much on the matter.
Not exactly next door to Ventura but if you don't find anything Bill Holland Cycles in San Diego is the local Ti god and has been building high end custom Ti bikes for over 15yrs and steel for 20+ yrs. Don't think he has a website but here's the contact info.
Bill Holland Cycles, Inc.
3735 Kenora Dr. Suite B
Spring Valley, California 91977