My weekday tallit is in need of new tzitziot as I am unsure how long they will last before becoming passul. I know that techeilet need to be made from a dye for which a minhag exists, but do the strings need to be made for the purpose of mitzvat tzitzit, or can they be any strings which fulfill the other requirements? (white/off white, wool or linen, et c.)
The strings for Tzitzit (whether they will be white or blue) need to be spun for the express purpose of use in Tzitzit. Some say the material needs to be carded for the express purpose of Tzitzit as well, but most are lenient about this.