Two good places, but call to see if they have Brie at the moment; though they usually do.
1.) Picnics Fine Foods
435 E 17th St
Costa Mesa, CA 92627
Hours: Tuesday to Saturday; 11:00 am to 5:00 pm
Closed Sundays & Mondays
2.) Laguna Culinary Arts Cheese shop
845 Laguna Canyon Road
Telephone (949) 494-4006.
Hours: 11:30am to 7:00pm Monday through Friday and 11:30am to 5:00pm on Saturday.
I'll second the cheese counter at the Whole Foods in Tustin, they are very friendly and will give you tastes and cut blocks to your size.
I have not been there (I am usually walking my dog so I can not go in), but there is a cheese shop on Glassel just north of the Orange Circle you might want to check out as well.