I think Uli uses mostly Autograph screws and bolts from what I remember. Without having checked their shop, I can't say if that is a) 100% accurate, b) if they are still available or c) if you might find similar replacements elsewhere. 3D-printing is definitely an alternative if you have a printer yourself and minimal 3D-designing skills. Or you can browse through the online catalogues for the larger online printing services (shapeways, frexion?).
If you are looking for actual metal pieces, you might also want to check out scalehardware. They used to have good offers but since management changed "recently" I'm not sure if prices have gone up or are still reasonable.
And of course there is always Top Studio and other manufacturers to provide you with bolts, rivets and other assorted bits and pieces. No links to those since I usually buy those in larger quantities via whatever shop is most convenient for me at the time. Ordering individual bags of 20 rivets from Japan to Europe just doesn't make sense
As an aside, I've added the links above, but am not in any shape or form affiliated with either of them. You can get similar stuff from other sources everywhere.
As for the part missing - are you referring to the white "triangular" shaped wheel hub? Maybe someone around here can cast a quick copy. If not, I would suggest contacting MFH directly (look around here, the Hiro support email address should be floating around this forum) via their email and ask for a replacement part. From what I hear they usually get back to you relatively sharpish so you can continue building. If contacting MFH I would suggest stating the kit # and also the part # from the instructions to avoid confusion
