Found this place that might be great. The moneys in pounds, though it ships to other places too, and does have PCI-E motherboards and cards and also allows you to customise almost everything you want in the computers (memory, graphics, motherboards, etc).
It also has many types of PC meant for Home/Office, then Professional use, etc, and then lets you customise components for each one, like a template in a way.
Seems very user friendly too!
It costs about £22 to ship outside the UK. Not bad really!
Check it out! I hope this helps