There's a Turner's Outdoorsman in LBC.
There's Riflegear in Fountain Valley.
There's a Ammo Brother's in Cerritos.
Go Go Go.
Just about anything 7075 aluminum is good to go. Be wary of plastic lowers. Post up your prospects, there are really too many to go through.
If you have any questions, the guys at Riflegear were very helpful when I was shopping for my first AR. I really like their Mod Aggressor multi cal AR15 lowers. $99, with flared magwells and tensioners. Ammo Brothers probably has the biggest gun rack in the area. I believe they sell stripped lower but I'd call before going. Turner's does have stripped lowers, but I'd call before going to get pricing and model availibility. There's a bunch in la county, and I believe you can order from any location.