If you're visiting LA and have a car... I'd definitely recommend Burbank House of Hobbies and Brookhurst.
If you can, do on a weekend - ideally in the mornings. You'll be able to avoid at least some of the traffic depending how far you're going.
Check what times they open online and be there early.
Brookhurst has a little of everything - new kits, older kits, armor/aircraft/scifi/ships/etc, large (but random) aftermarket selection and older decals too, good selection of paints and supplies too. The aftermarket boxes and decal binders are worth a dig through, you never