This is not a recommendation....
just answering the question, as far as I know.
Highway Sidecars, Mt Gambier SA.
SRK Engineering, Bathurst NSW.
I think everyone else is folded, absorbed, passed away.
having typed that....
+1 for Phyllis.
NB - sidecars are not cheap. You'll get what you pay for. Unless of course you are a skilled fabricator?
And you're arguably not paying for the 'pretty', you should be paying for the engineering. IMHO.
I read someone's sigline the other day. Something like, "If you think professionals are expensive, try paying for an amateur".