Ah, I think you're looking for rack mount shelf rails.
I also grabbed this one recently, but haven't installed it yet, so I can't vouch for it's performance:
http://www.ebay.com/itm/Adjustable-Rack-Mount-Server-Shelf-Shelves-Rail-1U-/261185735853?ssPageName=ADME:L:OU:US:3160These are static, but there are also shelves that slide out like the side mount rails would. Not sure which one you're after, but those type seem to be pretty expensive from what I've seen. (Generally starting at ~$100)
For the price difference, I'd personally drill holes in my equipment and pop-rivet some rails on there, haha.