I finished it a long, long time ago so I can't remember exactly, but, basically, it's in one single location in the fleet store. You get four chances a day for it to refresh unless you pay crystals for extra refreshes and it shares the spot with at least three other ships. So it can take a while to appear, but it's just randomness.
If you want my opinion, and of course you don't but it's the internet so I'm going to give it anyway and you can just skip it if you don't, it's all good, but in my opinion just save enouh fleet bucks so you'll finish Rex and then spend any excess on all the other ships. Get them all to 7* so that when you need them later you aren't at the mercy of waiting for one to pop in the store. Same goes for Guild Currency and characters like Jawa Engineer.