The Shulchan Aruch (Yoreh Deah 28:21) rules: “If one doesn’t have any dirt available with which to fulfill the mitzvah of kisuy hadam [covering the spilt blood from shechitah], he shouldn’t shecht.”
The Gilyon Maharsha asks on this din from a Magen Avraham and Taz in Hilchos Tzitzis (siman 13) who both bring a Mordechai which says, that if the strings of one’s tzitzis snap on Shabbos, he may continue to wear the garment. When it comes to the mitzvah of tzitzis, it doesn’t say, “don’t wear a garment which has four corners when it doesn’t have tzitzis”, rather it says, “when you wear a garment attach strings.” And since on Shabbos one is unable to attach strings there is nothing wrong with wearing such a garment. The Gilyon Maharsh adds: Presumably the same thing is in the week, and if one can’t get hold of tzitzis strings
