some standard placed in the states that are low quantity friendly
digikey
mouser
arrow
Newark
Jameco (I really dont like jameco anymore)
If you add Avnet Express you have a representative from basically every major electronics distributor in the business. I worked as an intern this summer sourcing electronics components, so the question where do you buy parts was basically my entire job. I often dealt with the higher volume branches of these same companies, but the parts they carry are usually the same. If you know what you are looking for one of these folks will have it. If you happen to work where I did, even if they don't have it they will bend over backwards to get it, but that is another story.
If you don't know exactly what you are looking for I would stick to the hobbyist sites, as they generally stock the most common and all around useful parts.