Their website allows you to search for parts, but often times you have to call to find out product status. Pricing at Arrow is really good. If you need a lot of one particular part, this is a great place to go. I order capacitors in the 1000's for cheap. However, there is quite a bit of time overhead required for ordering.
Arrow is also small guy friendly on the phone.
Allied Electronics Allied is another great place to find parts. They have a fantastic catalog that is worth getting. Their website is developing nicely, but is a little clunky at times. You can order online. They have great pricing, but have a minimum order size.