Armorcast and Anisty are good places to start.
Toys is not a bad place to start if you've got a knack for toybashing. I've done alot of it based almost entirely on cheap resale shop toys and the right toy can make for some impressive machinery without too much work.
pics and process here:
http://leadadventureforum.com/index.php?topic=70480.135One easy idea is to find some toy hull that looks like a box with lots of greebles on it as long as it has an input and output area, you've got some sort of "Machinery" ready to go.
Alternatively I'd look at 0-scale (around 1/48) model train machinery. There's all kinds of things sold for detailing layouts and for flatbed loads. 1/48 is more than close enough and most machinery is pretty scale-neutral. I actually prefer the slightly larger-than-28mm 1/48 for machinery and vehicles anyway.
You've got to do a big of digging, but this guy sells some stuff you might be interested in.
http://www.rustyrail.com/RailroadCarLoadsOScalePage.htmNo assembly lines, but he has various mechanical things, boilers, etc.
There are also some Transformer (electric station, not robot) kits in O or HO scale that might work well also.
I have this kit and it looks great alongside 28mm figs even though it's only HO (1.87) scale.
http://www.hobbylinc.com/walthers-transformer-kit-ho-scale-model-railroad-building-accessory-3126?source=froogle&gclid=CNPZt8yYr9ECFUIvgQodB_YA3QThis guy has amazing O gauge kits, but they're also incredibly expensive:
https://www.sierrawestscalemodels.com/index.htmlLastly, they may be hard to find, but the Pegasus/Tehnolog "Power Plant" and "Chemical Plant" kits are chock full of industrial goodness. Here's a seller offering them by the sprue:
http://www.ebay.com/itm/Chemical-Plant-plastic-terrain-set-main-pipes-sprue-/141946319870Not quite the deal they were in the original boxed set, but still cheaper than most othe options and very easy to use. I've used these bits on many projects.