Where in campaign or elite campaign can I farm/sweep flax material if possible or where on map can I obtain it?
First of all you should never use your energy to farm materials. Always use your energy to get soulstones for heroes, so you can upgrade your heroes to higher grade.
When you go to your smithy > warehouse > materials and fusions, you can click any material and the description tells where you can obtain that particular material. You can obtain flax from harvesting meadows.
Also, do not make duplicate threads on forums please.