I'm not sure it is the one you saw, but McMaster has slotted angle and flat stock...
... and almost everything else in hardware. Good source, but not always the best price.
All the usual disclaimers.
Make it fit.
slotted bar stock
A few months ago, I found a website for a company which offered bar stock in different slotted configurations, in 12' or shorter lengths.
Does anyone know of this company? I can't find them by searching. I can't remember where I found the link. I keep thinking the name had something to do with Kentucky? Anybody, any ideas?