The video card you've already got would do a good job keeping up with the HD 7790, so upgrading to the new card won't do much for performance. You could go two ways here - upgrade to the HD 7790, and sell your card - trying to secure as much money as you can to put toward another HD 7790 and CrossFire those GPUs together.
Alternatively, hunt the Internet for a second GPU identical to yours and CrossFire it with your current card. You've asked where to find one, so I'd suggest the Overclockers Australia forums. They have a Shop & Swap section, where you could either sell your GPU - or put up a Wanted to Buy ad for another GPU identical to yours.
If that fails, then reconsider your options again and maybe sell your card to raise some funds, and spend the $50-$100 you'd get for your GPU and put it toward a HD 7870 or HD 7950.