Give this one a shot:
It's a Koolance Res/Pump designed for DIY systems like yours.
If you just need a Reservoir...
I've seen a number of people use EK Waterblock reservoirs.
That's the link to their reservoir page on their site, choose the one that best suits you. Prices range from 29ish to around 60 dollars. ((Attempt to find an American Distributor of these reservoirs, as they seem to be based in Europe)).