If you are already using poles and getting benefits from that, then I would recommend that you get a reasonable set on arrival. The nature of the terrain is generally irrelevant - there might be some additional benefits in difficult terrain, but I have rarely seen anything so difficult that poles were 'essential'. Rather it is the ability of poles to reduce the load on lower joints, improve one's balance with a pack, etc. These will be there just as much on the flat as elsewhere.
On the matter of where to buy them, my experience with really cheap poles is not good. I bought one pair that lasted all of two days before they collapsed. I bought a mid-range (~30 euro) pair in Melide that lasted well after my return home before one of the closures failed. All poles will eventually fail in one way or another - the trick is to avoid something that won't last the distance.