They're called mud plugs. If you look on the Yoshimura website and go to the Suzuki LTR 450 section you'll find them. They're like 18 bucks. Hipertech also sells them but the design might only fit their rim--not sure.I am trying to find the plastic caps that knock into the centre of an 8" bead lock to stop them filling with mud.
I don't know what they are called & am having no luck finding a set.