We often get asked where to buy 8020 extrusion.
This really depends upon where you live. OpenSimRigs can supply 8020 to anywhere in Canada at a competitive rate.
The largest supplier is 8020
eBay off cuts
Minsumi in the USA is a good alternative to 8020.com
McMaster in the USA is another great supplier.
If you are looking for small section extrusion for accessories, then OpenBuilds.com offers some great extrusion and gussets.