The way I see it, you have four options.
Option 1: See if you have a techie friend/family member who would be willing to assemble your rig for some favors. I personally wouldn't mind building someones else's machine, building is fun.
Option 2: Get your components from a local supplier, chances are they will have some service where they will put it together for you (Will likely cost a lot though).
Option 3: Buy a custom system off CyberPower or NCIX US, I think they allow you to select custom components and will ship the system to you mostly pre-assembled (again, will likely cost a lot).
Option 4 (The best in IMO): Do some research and build it yourself.