I looked high and low and could not find any side case racks for sale, anywhere in the world, new or used, on any forum or eBay. They're long since discontinued, so if you want to mount Givi side cases you are out of luck.
But.. I really wanted a pair of top-loading E21's, so I took a gamble and bought racks for a Yamaha FZ6 since they looked pretty similar to the 919 racks. With a little massaging, they will work. I make no claims about the longevity of this setup or how pretty it is. I'm posting merely to show one alternative to giving up and using soft throw-over luggage.
The main downside to this setup is that the racks don't sit flush with the top case rails. You'll see in the pictures that there's an angle between the rails and the racks, but it actually feels a lot tighter than it looks. You still use the original hole in the top rails, screw it in nice and tight, mount the lower bars of the racks to the passenger pegs which is also nice and snug, and then there's a bar under the tail that connects the two sides and holds them together. Again, nice and snug.
First off, you need to mount the rails that hold the top case. Whether or not you use these rails for an actual top case is irrelevant, but they are needed to attach the side racks. I found a guy selling the rails, top plate, and huge top box for $60+shipping on eBay. A real steal.
Here are the top case racks, still available from several retailers: http://www.twistedthrottle.com/ezima...86-500x600.jpg
Next, pick up a set of Givi PL360 hardware. This is the model made for the FZ6. I found a set on eBay for $75.
Finally, mount the bags you like. I chose the E21 top-loading cases. Why anyone would want side-loading is beyond me.
Anyway, the PL360 kit comes with everything you need including spacers that can be re-purposed to make the under-tail stabilizing bar long enough to connect the two side racks. You also need a hacksaw and a drill.