Ah okay, I'd highly suggest you build one as you get best value for money and there's a ton of help guides on toms.
but if you want to buy a prebuilt one look at these kind of places(they make custom pcs)
http://www.ibuypower.com/
or
http://www.cyberpowerpc.com/
You'll probably want an i7 4770 with 8GB RAM, R9 280X or GTX 770 4GB and then you'll be able to record in good quality.
I still suggest building over buying custom unless you're VERY worried about building.
Here's a special offer and it comes with a customiseable setup you can play around with.
http://www.ibuypower.com/Store/Intel-i7-Paladin-Special
post back what you've chosen.