Just picked up one up from our local rack shop. Walked in ,ask for a handlebar strap, the guy reached into a drawer, grabbed one, I gave him $10 and walked out a happy tandem owner.
Ours disappeared a couple rides ago and loading the bike without it lead to chips in our new paint job from the bars swinging around out of control during loading/unloading.
A simple device that I can't live without.
Good luck and if you don't find one let me know, perhaps I can help.
KRhea
PortlandVelo.net