You have to make up your mind which part of west coast you will be, LA area, SF bay area or Vegas. Because they are hella far apart. California is huge dude. So due to this I can only recommend chain stores to you.
You can try
Wolf camera.
Another place we go a lot for electronic items is this electronics supermarket called Fry's. They are located all over the place on the west coast.
Store locations are
HERE.
In US, I usually buy online because prices are much cheaper. Problem is, you have to be careful what you buy. If you got a Grey set, you will not have warranty. And you want to give yourself enough time to do a return or exchange should there be a problem. If you are just there for a short trip (less than 60 days), and if you have to move from place to place a lot, online buying is probably not an option for you.
If you are buying kits like 500D kit, or D5000 D3000 D90 kit, you can go buy it cheap at
Costco. But being a members only discount hypermart, you need a membership card to purchase anything in there. You will also have to do CASH or American Express (if I remember correctly). You can also do debit card (like Netts in SG) payments in there if you have a USA bank account. Visa, Mastercard not accepted.
If you can tell me your exact location, then I can help you more. And, just saying SF area or LA is not enough. These areas themselves are huge. You have to go down to county or city level. For example. San Jose, Fremont, Westwood etc etc etc. And I hope you are driving because, apart from Vegas, taxi cabs are nearly non-existent in these places.