Check craigslists / kijiji for the GPUs. They will be the cheapest option right now but you need to do it before everybody catches on to the ETH mining and they will be sold out. $150 I paid for a few 7970's, $170 for a few 280X, etc. Couldn't get them any cheaper. Some people really think their GPUs are gold and are selling them at the same price as a brand new 380X.
The motherboard and CPU you might as well buy brand new, they are cheap and you can always resell them at a good price.
Risers you need to buy on eBay. Make sure they are powered with the +12V/GND lines cut from the mobo end.
To mount everything on? Just buy an adjustable shelf, you can get it at CanadianTire or HomeDepot, around $20.
If you find me some cheap GPU's i'll give you a cut.
I already have a solution for mounting, i'm just lazy and would like something more compact. If you have a link to show me what you are talking about, that would be nice.
For risers, normally i get them for 13CAD ea from Bitcoinware. I checked ebay and saw much higher prices.
I might do that for the mobo kit, i already have an order i can place anytime, just waiting to see if i can source GPU at a good price first (and might as well fish for better Mobo/cpu/ram deal meanwhile, since newegg will take 2 days tops for me to receive the mobo stuff), and on Kijiji it can be annoying because i dont have a car and dont trust people to ship.