Is cotton with polyester allowed to be used for Tachrichim?
The minhag is that the tachrichim and the threads used to sew the tachrichim should be from linen. (In kabbalah there is a certain protection to the person if dressed in only white, linen garments). If this is not feasible, or too expensive then it is permitted to use cheaper fabrics.
Rambam Hilchos Evel 4-1, Livush Y:D 352-2, Pischei Teshuva Y:D 352-2 in the name of Yalkut Reuvaini, Gesher Hachaim 10-1.