Ok... first thing - RO
units don't produce water on demand, so even if you got it up and running right_this_minute it's not going to do your rock any good. So no need to stress about the immediate need for the RO
unit - if you didn't have the saltwater ready when you got the rock, you were too late to start with.
The rock isn't junk. If you keep it wrapped up in saltwater soaked towels, a fair amount of bacteria should stay alive for 12-18 hours at least. You may have a little ammonia spike when it finally goes in the water, but probably not much.
If you let the rock dry out, or put fresh/tap water on it, then your bacteria will die off. But it's still not junk... you'll just have to cure it a little. If there's nothing in your tank, you can do that in the tank when you get it filled. I'm guessing that's the case because if you don't have saltwater around, I'm thinking you have an empty tank.
Yeah... you can use distilled water from the store to mix up your salt. You could use tap too in a pinch, just make sure you add a dechlorinator to the tap water per the instructions.