I am very confused.. no one mentioned what this is all about, is it for a custom air filter? is it for the exhaust?, is it for intercooler piping? or is it to swallow cold budweiser fast as a funnel while watching the Superbowl.
If you could be more specific I could direct you rapidly to the right website but I am still guessing what you are trying to do with a "coupler".
Anyway, let me help you out, go to
www.extremepsi.com and look under their DIY section, they have all kinds of couplers. Why does it have to be aluminium? can it be just regular steel? Again, I am lost here,
what exactly are you trying to do? Don't worry, no one will copy your idea. Here is another website that has couplers
www.forcedperformance.com, look under their DIY section