I contacted both the Canadian (Lambert) and American (Topeak) distributors and neither of them have the raincover available for the Trunk bag (mine is the DXP). The American distributor said that they may end up getting them in June (now) or July from Taiwan. Other than waiting, they have them here:

Allterraingcyclys
Wiggle

I may end up ordering from one of these sites, I will contact Topeak again before I go ahead and take the hit on buying from England.

Good luck with the search though!

Kerry