I have made all my poufs using industrial grades wool or wool-mixes. I have bought them all in the salvation shop or at factory outlets and they had no brand-names so I cannot share. But what I go after is "hard" wool, wool that has not been through softening processes and is maybe even mixed with some polyester to make it tough. I think I have used carpet-yarn, furniture-yarn or weaving-yarns. I use yarns that knit on needles 3,5mm - 5mm. I promise that if somebody finds a brand-name that I will share it with you all here.
If you cannot get hold of this kind of yarn or you are lazy you can also buy very thick wool (rather expensive) or cord and consequently skip step 2 - making a cord. The Norwegan Pickles sell a very thick yarn that can be used for a pouf; Pickles fat and happy.
You can use Zpaghetti, cut up old t-shirts yourself, or buy tube-yarn or felted cord. The last you can get
at Etsy (handmade) or as industrial grade felt i.e. at Filtmaatschappij Rotterdam (this is where Christien Meindersma got hers, I think) You can also use woven (cotton) cord i.e. from the Hema.