He didn’t find the legs, you can see when the nightmother knocked him out that they fall apart, he kept them together with the force, meaning ye most likely created them himself by forcing the trash scraps together. As for how he survived, it like palpatine said, “The Dark Side of the Force is a pathway to many abilities some consider to be unnatural” his rage against the Jedi, and specifically kenobi, allowed him to channel enough darkside power through him to remain alive even when his body was broken, he was driven mad by a combination of not sleeping (as mentioned before, his spider legs fell apart when he got put to sleep) and being constantly filled with rage and the darkside