Every character on Jiǎoluò Shūwū read as zi in any tone — 3 readings total across the four tones. Pick the tone you mean.
If you have the sound zi but not the character, here are the 3 characters read this way: 子 (zǐ, child, seed, Master), 字 (zì, character, written word), 自 (zì, self, from, naturally). They split across 2 tones, so the tone is what separates them.