Edit:
. Ninja'd.
That DDC Res top won't fit on the CPX-Pro.
I highly suggest a GTZ instead of a GT, try emailing Linus to see when they are getting more, or there is a GTZ in the Buy/Sell/Trade forum for sale.
Reservoirs -
NCIX.com - Buy Swiftech MCRES-MICRO REV2 Clear HI-FLOW Water Cooling Reservoir 3/8IN & 1/2IN Barb Fittings - MCRES-Micro Rev2 In Canada.
or
NCIX.com - Buy Ek Water Blocks Ek Multioption Res 100 REV.2 Water Cooling Tube Reservoir G1/4 - 3830046994165 In Canada. Both will take regular 1/2" barbs.