I have worked with dozens of agencies, sourcing and collaborating with designers, as well as sourced and hired for my own consultancy.

I agree with the other answers here in regards to what great designers are looking for. Money is a factor but not usually the driving factor. Opportunity to be creative, use new design techniques/technologies/trends, and solve problems are the top motivators for web/UX designers.

However, the other answers don't really address where you could find them. You mentioned that you've tried sites like Elance and Guru without success. The majority of service providers on those sites are average at best, but there are definitely good and great ones there as well. Consider changing your approach. In your messaging and positioning, use the advice from the other answers here in terms of the opportunities for interesting, innovative work. In your reach out, rather than just putting up a listing and hoping for the best, seek out individuals who have great reviews and portfolio examples that you consider to be top notch. Reach out to them personally and describe the opportunities you have available. Start that conversation to gauge their interest and find out what are the top motivators for each individual, and subsequently, show them how you can fulfill their needs. In most cases, people who are at the top of their craft have their pick of who to work with. You need them more than they need you. So it's really about showing them the benefits of working with you.

In addition to freelancer sites, there are lots of ways to seek people out in the offline world. Figure out where great designers can be found in your city. Meetups, conferences, co-working spaces, etc. Go there and talk to them one on one. Nothing communicates passion and opportunity like a face to face conversation.