How to find PhD students interested in 6-months paid exchange visits to work on a supervised project?
Your post has hit on the problem: you want some PhD students to come work with you for six months. This is a bit too long for a summer internship-like posting, but not really long enough to justify the work required for what would amount to a relocation. It's like a long temp assignment. 

Perhaps you should be looking at trying to support some students who are just finished with their PhD's—or have just handed in their theses and are figuring out what to do next—at nearby universities. Or trying to arrange the funding to support a postdoc for a year.