While not the larger, fullsized sidecar hacks pictured in the first post, nor the smallest, "minimoto" sidecars pictured in Defector's post, a collection of the largest group of racers and fabricators of sidecars that slot somewhere in the middle (yet are still referred to as "mini") can be found in the UK on a forum.
These are SERIOUS sidecar guys (and gals), and they could probably put you in contact with valuable build info regarding making a hack of your own.
http://www.minimotoclub.co.uk/vb/forumdisplay.php?f=24
Good luck. BTW, sidecar racing is the second most fun thing you can do that involves your GF (if she's crazy enough) and leather.