On the Barnett site..click on cables (on the left side)...this will bring up clickable pictures of cables..click on the one you want then from the drop down put in your bike...I think you said 2010 America

http://www.barnettclutches.com/1982/triumph/0/0/2010-triumph-bonneville-america.html

These are the stock cables..so with the Barnett part number say "Part# 101-75-30020" you would order part Part# 101-75-30020 + 3" ...that would be Throttle Cable 3" longer than stock.

Having these numbers you can go to a local Harley shop (not a dealer) and they can order the cables..Also at most Harley shops they have break line fittings and hoses. You can make up any size break line you need...

Last word of warning is the Harley shop will say..."We don't carry Triumph parts"...Just tell them you have the part numbers and not the worry...

One other idea is to use tie wire, I think that is what it is called (soft cheap wire used to tie re-bar with), and run the wire from the handlebar to the carb or what ever the same path you want to run your cables...measure the length of the wire...that is the cable length..You can also mock up a set of handle bars by using 1" PVC pipe and fittings...you can get rise and pull back numbers and cable length..(done the last one before I did my last set of bars)