The bolt is a M10x70 (12) and the nut is M10 locknut (13)
http://www.bikebandit.com/2010-triumph-america/o/m18745#sch631955The risers usually come with the bolts to install them. If you switch to Harley they are Standard inch size bolts..I would guess around 7/16" bolt which is just slightly bigger than a M10 (10 mm bolt).
If you stay with parts from British Customs
http://www.british-customs.com/ or Fast Eddy
http://www.fasteddysports.com/ or New Speedmaster
http://www.newspeedmaster.com/index.htmlThey all have risers that fit the bike with no mod's...