I'm sure there are plenty in LA outside of Hollywood but since thats the only area I know I can point you to a bunch on Sunset Boulevard. Guitar Centre on Sunset is pretty big and has the Hollywood rockwalk attached which is interesting. There are many others along sunset including the Mesa/Boogie outlet store which has a delicious collection of guitars and amps. Sunset is quite long and spread out though and I would drive rather than walk.
In NYC everything is in 48th street near times square. Mannys, Sam Ash and Rudys are 3 of the big ones that you'll find, all next to or across the street from each other.
For studio gear try BH Photo Video on 9th street (but check the warranty of anything you buy, a lot of grey market goods with no warranty). There is also a nice used guitar shop on Bleecker in Greenwich Village, Matt Umanov rare guitars.
Ahhhh...wish I were going