well in my experience, music shops will not always
stock such accessories as quite often they don't move all that many. Most common ones that I see are for rack mounting half rack sized equipment or for wireless mic receivers.
If they
stock Peavey, then they should be able to order it in for you.
Otherwise, just fabricate some out of aluminium angle - you should be able to get it in the necessary width. If not, get some sheet metal bent up.
* Cut to length (make them the same length as the side of the
mixer as this will make
lining everything up a whole lot easier).
*Hold each length behind the rack strip to get the right position for the rack holes.
*
Mark out the holes to mount them to the
mixer (use a cardboard
stencil to do this).
* Drill your holes and screw it all together.
Maybe 15 - 20 minutes work.