I would suggest using PC Partpicker if you know the parts you want. From http://pcpartpicker.com/ go to either Start A System Build or Browse by Individual Parts. After selecting a particular part, you can see how much it is from where and what offers are available (shipping costs as well).
I bought some of my parts from Amazon through their Amazon Warehouse Deals (open box items from the Used section on said part). I bought items such as my 970 EVGA FTW card that way and got it for about 309$ at the time where a brand new one would have been about 360$.