I just went into the world with a quick tester airline.
You are able to order any of the aircraft, the old or the new, from the factory. There are none used, but that does not mean there is not the ability to get them.
The biggest operator of the CRJ 700 NextGen (a/c type 17210; not listing player by name) seems to have a medium age above 5 years, implying he got at least some of them used. And a quick look at fleet lists checks out my theory, with that operator having used a/c, planes from Jul2015 to Dec2020. Two of the smaller operators have one plane older than 6yrs old ea. So there are some - but not a whole lot…
I believed the original list of used aircraft to some degree they’re based off fleet lists from IRL productions. Not sure that is true whatsoever still.
But you’re more than welcome to order them from the factory. If you’re a used market purist, however, then you’ll have to hope one of the operators ceases. Usually used aircraft are not added after a launch. No one else seems to have ordered any of the other 5 types of CR7 (17200, 17300, 17310, 17320, 17330) except for one other operator.
Hope that answers your questions. If not, then let us know what else you need