Critical Mass do to 15mm grav bikes (in their Arc Fleet range), and I suppose a torso swap with the Thrainites might be quite passable.
CM also do some Mercenary Star Marines that could work for Exo Armour if you cut the legs down a bit (should be simple enough to do).
Rebel Miniatures do some post-apocalyptic bikers that could be a good source or Harley-style bikes.
Ion Age have some nice-looking tracked bikes
here. The also have a few rather nice-looking battlesuits too.
15mm.co.uk also have some interesting stuff.
This jetcopter might suite with a little work perhaps? They also have
a bike that might work if you're after a more classic Citadel look. Finally, if you're after some irregular-looking trrops, they offer the
Niflung troops.
I think some of the WW2 ranges might have suitable bikes/vehicles that could be ported over with a little conversion work.
Anyway, that's my first stab - I'm sure that folks with more 15mm model experience than I can suggest more.