You can get clusters using your level 50+ miner/botanist in Mor Dhona. They spawn mining: 9:00 - 12:00 1:00 - 4:00 5:00 - 8:00 ET Botany: 13:00 - 16:00 17:00 - 20:00 21:00 - 0:00

It would be worthwhile searching out "Teamcraft" on google, that has a great searchable database of everything crafting related.