Where to buy a dual-CPU linux workstation for HPC? (ideally in Canada)

You're probably looking at a Supermicro system if you DIY. Otherwise you could try the business line workstations from the normal OEMS.

http://www.supermicro.com/products/nfo/superworkstation.cfm

Warning though -- some of those supermicro systems are very loud, and rival similar 4U rackmount systems. In fact, most of them have rack mount conversion kits available to be loaded into a server room.

You could buy the parts and build it yourself. You could find somebody local to assemble you one. You could order one from Dell or HP or Lenovo -- at least Dell seems to have a reasonable markup over parts cost, I haven't looked at HP or Lenovo.