I am trying to design 500W PS , the input is 208 Vac and out put is 24VDC ,isolated from input, which topology is low cost and less components and can I use flyback?

 

Designing such a thing will take months.

You can buy one for cheap.

 

I am trying to design 500W PS , the input is 208 Vac and out put is 24VDC ,isolated from input, which topology is low cost and less components and can I use flyback?

I don't see the need for a flyback design, but you can do anything that you want to do. You certainly cannot do it cheaper than buying one.