here what I know:
dbilas sells the intake manifold, the one in this pic, between the ITBs and the head for m42 and m44 separately from itb, if you need it:
http://images41.fotki.com/v1307/photos/4/41655/5668144/DKA_BMW_M40_M43_1_6_1_9_001-vi.jpg It will cost you something about 296euro. It's flanged for DCOE carbs, as their throttle bodies are.
You would need to pick up some throttle bodies with DCOE flange (extrudabody, TWM....), bolt them to their manifold and build a custom airbox.
This way it could be cheaper... and it could be cheaper too if you can find used ITB with that flange, not too impossible.
EDIT: cheaper if you have a tig welder... you will do your own inlet manifold for ITB...
do not think that fitting ITB only you will gain a lot of power. If you want real gains as natural aspirated you MUST to open the engine.