We've had many requests for these over the years from Thule rack owners, due to lost/broken/rusted hardware. We are offering a package of four M6 x 80 carriage bolts in stainless steel, which means they will never rust again.
 
We are not Thule experts, so please verify these are the correct ones for your application. The M6 thread is slightly smaller than 1/4" in diameter, and these are the coarse pitch (1mm), which is the most common.