What is the best website to place orders for electronics components online in the USA?
There are many good companies and you will grow a favorites list with time, but I (in New Zealand) find Digikey very good www.digikey.com.  They have a vast catalog range, much is in stock, prices are usually in the very good to acceptable range. Delivery is rapid and they tend to pick and dispatch same day in almost all cases. Do not be put off by where their HQ is :-).

BUT look at Findchips which will give you some good ideas. 

RS are the masters of a vast stock and usually premium prices,

Farnell are a successful RS wannabee.

Look also at Mouser, JameCo, Amazon, ebay !!!, ...

Findchips supplies information for these suppliers: