Well, unless you can find a board that has the exact part you want, you don't have many options:
1) Design and build your own board. Depends on your skill level and time frame. Not for beginners.
2) Buy an existing development board with the 5M570Z part (in 256p BGA I believe), buy 5M1270Z parts, and have a rework tech or shop pull the existing part and populate the new part. Depends on the resources you have available. Will cost several hundred US$ if you need to have a third party rework shop do it.
3) Switch to a similar architecture part on a board that is available off the shelf. Ie, look for a Max10 based solution.
4) Scale down your project so it fits in a 5M570Z part.
Not a lot of other options. Depending on what is most available and important to you (time, money, effort) those are your choices.
More thoughts:
5) Use two (or more) 570Z boards and spread your project across them, either bit sliced (horizontally) or pipe-lined (vertically). Depending upon the nature of your project (which I have no idea of what it is) either of these options may or may not make any sense.
The 5M570Z board has the two 40P GPIO connectors with 36 signals each, exact same pinout. So you could jumper one board to another and configure a 32b or so data port between the boards in a two stage pipeline. Or even three (or more boards) daisy chained. Just a thought.
