Hello and Welcome!
Darth Nihilus is one of those costumes you will have to look around to source all the various parts. As for robes, tunics, obi and belts, I do take commissions, but I am booked till March 2016 currently. So if you need it after that, you can PM me. My Wicked Armour does make a Nihilus, but it needs alterations to be 501st approvable. You can also approach a local seamstress or tailor and see what they would charge to make it for you. The best pattern to use is any of the Jedi ones, but they will need to be altered slightly. I used the 5840 Simplicity pattern for mine.
For fabric you will want something with a good drape to it that hangs nicely. Many use Raw Silk or Linen for their robes. I used a heavier piano weave linen for my robes and it is gorgeous, but also pricey. Plain black quilters cotton is too light a fabric to get the desired effect of Nihilus' billowing robes.
For gloves, I found mine online at a good price. Any black, non-stiff gauntlet glove, without stitching on the back, can be used. Boots just need to be black and have no visible laces. I use womens black zip up snow boots on mine. Not ideal when trooping in the summer heat however, hehehe. Hope this helps! Cheers, Nef