I did a little searching and found that a couple places do have ammo that you can purchase online, but it looks like a lot of places are in fact out, and those that have it are priced at almost $.50/ea. Considering your other choice is a .308, which has similar ammo prices, it's a bit of a toss up. With that said, in shooting a round that has limited local availability, it might be a good idea to get into reloading, mainly progressive reloading (eventually) to feed your habit and not have to worry much about not having ammo. You'll also be able to save a lot on the cost of the ammo as well, although it'll be a higher start-up cost.
I think it's really up to you to decide if it's worth it to wait, depending on how much you like the firearm itself. Personally, for toys that shoot a lot of rounds in a short amount of time (on the cheap, too) I'd favor an AR-15. You can get a basic one in 5.56/.223, then get a .22LR conversion kit, and if you decide later on you'd like to use the 5.7x28mm round, they make AR57 uppers.
If it was my choice between the two, it would be the PS90 hands down; but I also reload, so as long as I can get components I have ammo.
