I got a shedload of buckets, powder barrels and little pyramid piles of cannon balls from Bicorne when I bought various ECW guns and crews from them.
Ditto The Assault Group 30 Years War guns.
Don't know whether either of those two manufacturers sell all the artillery set dressing separately, but they're all nice pieces sculpted in both cases by Nick Collier. Work perfectly well for WoTR.
Would be worth checking their websites.
Maybe a Perry plastic late medieval artillery set will happen one day. Feel sure that would include such bits and pieces. There again, it might never happen and isn't going to help you now