Hi Tuor and Mith,
Agree that the East-West Dorwinion-Erebor-Lindon traderoute was significant, North-South there seems to be very little evidence, apart from Shire-Orthanc under Saruman, doesn't mean that it didn't happen however.
I think one plausible mechanism is one that's been proposed for bronze-age Europe, where occasional Greek artefacts turn up in Britain etc. This is the chain of trade, so for example an item made in the Shire could be sold by a hobbit in Bree, picked up as a curiosity by a trader at the Prancing Pony, stolen by the Dunlendings, given as tribute to Saruman, sold on to a Rohirric trader by one of Saruman's agents, who throws it in as a sweetener for a horse deal with a Gondorian, who sells it in the market at Minas Tirith to a noble who thinks its amusing, who notices the Steward looking at it and gifts it to the Steward as an outlandish curiosity.
Hey presto - Denethor with an Umbrella
