Exactly.. what you are asking for in terms of "Interns" are what are normally high-paying professions... in consideration that the fashion world places values of hundreds of thousands of dollars for a single outfit, these "interns" of yours (professions which take years to master) are being compensated, and commensurately, right?
You are asking for highly skilled professionals- but if these "interns" envision, design, and build "your" creations, whose name and credit will be on it? By defining that professional as an Intern, you from the outset make it sound rather..sketchy.
There is a level of experience that no longer puts a person in the "Intern" category, and I think you may have a hard time getting a worthwhile respondant who isn't expecting to be more than an "Intern".. in the fashion world, it's all about two things: Name and Money. If defined as an "intern" your prospective applicant might reasonably expect to see neither. It's maybe just a name thing, but if that's the case then you really ought to make clear that you are looking for a Partner, rather than an Intern.
The skillsets you want won't likely be had for free, that's all..