Moto Grand Prix
Main Motorcycle: '02 CE Speedtona 955i
Join Date: Nov 2006
Location: Buffalo, NY, San Francisco Expatriate
The best places to get larger sockets are at flea markets, swap meets, or cheaper tool stores. Craftsman sockets at Sears are sometimes the cheapest way to go, depending on where you live. Checking eBay may also reveal some deals.
For what it's worth, I've never used a torque wrench on the steering head adjuster nut or the lock nut. 40nm is nothing, and this can be roughly gauged by applying 30 pounds of force on a 1-foot-long wrench. The same applies for the final 6nm. 6nm is so light that once you feel resistance, stop. Again, 5 pounds of pressure on a 1-foot-long wrench. You will know if it is too loose, so the biggest worry is overloading the bearings. Luckily, the steering head bearings are not rotating at extreme speeds, so if there is ever a ware problem, it is usually noticeable before a catastrophic failure. This, of course, is assuming one does not severally overload or underload the bearings.
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other. -Macbeth knew I.VII