That's really unlucky! Haven't heard of getting same sides before, but both my brother and cousin have got different size boots before (i.e. left boot size 9, right boot size 11) - both at warehouse sales. Either the salesperson was careless or they weren't paying attention.

My brother found out as soon as we came home, cause we were playing that afternoon, so we went back and got him the right size. My cousin wasn't so lucky. He only found out a week after.

My suggestion is to check small stores - maybe they have display models that have lost their partner. Also, what type of boots? Kinda hard to find the same style/color if they're older! Good luck!