I understand that just buying one side skirt is going to be very expensive (since they are sold in right side/left side kits). Can yours be repaired by a body shop, or better yet, do you need a skirt at all or are just looking at needing clips. If the latter, the following might be helpful:
Do you have the Mazda ones, and if so what year. If you have the OEM and your car is a 2014+, here is a thread that might be useful (specifically page 2) OEM Side Under Skirt DIY: 2014+
It lists the parts for those skirts, and those clips are often bound at good auto parts dealers, body shops, etc. If me, I would take one clip that is not damaged (or lightly damaged), and visit a few of those places. Or take that clip over to a Mazda dealer.
A dealer will not just sell you one side (and I am guessing you already know that), and according to my Mazda dealer, OEM side skirts are rare (they sell just one set a year), but a very long shot would be to go to a auto junk yard, and see if they have one. Modern junkyards are mostly computer interconnected with even their competitors, so maybe one Mazda 3 with a OEM got into a wreck, was totaled, and yet that part survived.
Just shooting ideas off the hip and hopefully wil work for you. Best of luck Matt!