I don't believe they've ever put any Race Against Time skins in the Premium Store, literally the only way to get them is just to be patient and check each week (hell I'm still anxiously awaiting the return of 'La Cienega' Cassie Cage to more closely match my similarly dressed Jaqui sporting 'Double Tap').
That said, in the meantime, you can far more easily unlock the 'New Sensation' skin (has a extra few white elements that were black on 'Box Office Champ' but otherwise identical and exact same colour scheme) from the Towers of Time entitled Family Outing, or the 'Cutting Glass' skin (a few dark green bands that were white on 'New Sensation' and black on 'Box Office Champ', but fundamentally looks like classic MK2 Cage as the green blends very closely with the blue) from the Krypt as a Kollector's trade. It's one of the few classic skins in the game that at least has two REALLY closely coloured alternatives you can use whilst you wait for the one you want.
Hell, I unlocked 'Box Office Champ' years ago, but I actually use 'New Sensation' on my classic Cage as I prefer how it pops more with the extra white.
