Boye cables including the interchangeables are known to be hard to work with. They aren't bad for flat knitting, but almost impossible for some circular knitting like magic loop.
Knitting Help sells Knitter's Pride sets that are great and the cables are flexible. They also sell Denise, but I'd recommend the Knitters Pride over them. I suggest getting the Comby sampler which gives you the 3 different types then you can choose which needle type you like best. My personal choice would be The Nova metal or Dreamz wood. Since you are new to circs though the sampler is a good idea.
The tab labeled Shop at the top of this page is where you'll find them.