I can't give you an accurate answer but judging by the GTX4xx series i'd say it will be easier to get a GTX560, even now there is a much greater diversity of 560's than 570's, probably because it sell better than the 570's due to lower prices while still having a good performance.
Also, you will need a pretty good processor to get the full potential out of two 570's. What is your CPU?
As i said, it will probably be easier to get your hands on a 560 in a year or so but you will, almost for sure, be able to get another 570 and if you can take full advantage of two of them than i suggest to get one now, not to mention that you will be able to play games on higher settings in the meanwhile.
Also... what's your power supply?