For the question of where to get them I know a while back the NIS store adopted free shipping for all their products. (On top of their already low prices) If getting a physical copy if what you want that is the way I would go.

If you go for the PSP you can also download them from PSN. (Note, Disgaea 2 is only available for the PSP and PS2, not the DS)

I would personally recommend getting both 1 & 2 for the PSP. But that has more to do with my feelings on the PSP and DS, not the respective ports.