Neither Akula nor BOF sell their Dune inspired figures now (or at least not as of a couple of years back when I looked) so you will be in for a lot of converting or hunting on the resale market.
15mm is easier, Rebel make figures suitable for Fremen, Khurasan do suitable Harkonen with characters from MJ, vehicles from GZG or Khurasan, Worms from Irregular or D&D miniatures if you want rearing up types (big enough for gaming but not anything like the size of the book ones).
Quite a lot of choice if you hunt through teh various ranges, none of them actual Dune figures (unsurprisingly) but close enough to recognise what is on the table.
edit: Forgot Atreides from GZG (sorry Jon!)