Fooling around with sidecars is so small a niche that I would guess 90% of rigs are put together by some local guy with a welder and a few pictures.
The gas cost and hours spent driving my truck and parts up to the nearest real sidecar shop is greater than the total cost and time I have in building the bracketry myself.
To me, putting together myself is a major part of the attraction. My next build will be a Harley with a "Woody" hack. It's just a matter of time and $.