Unless you absolutely need isolation, I wouldn't go for a flyback design. You'll need a handful of components including a transformer that can be costly (unless you're targetting very high quantities), and designing a robust flyback converter is not as easy as it sounds. If this is a learning exercise, then why not, you'll certainly learn a few things, but expect to be burning a few transistors before getting it right. And I'm not talking about EMI...
There are high voltage buck converters, mariush gave you some references, you can also find some from LT/AD (for instance, the LTC3894, but there are a few others in the same ballpark depending on the output current you need).
Just my 2 cents.