我已经有一个汉语老师了，但还想找（一些）粤语的老师。 了is for past tense in Chinese, although the usage of it is complicated in terms of whether it is necessary in a sentence. 已经…了can be used for perfect tense. As for another half of the sentence, it's more idiomatic to add a 还（still）， besides I feel weird about some teachers though it is grammatically correct, in Chinese singular or plural can't be recognized by the form of the word(unless you add “们”）， so you may as we'll omit the 一些。

我已经有一个汉语老师了，但还想找个粤语老师。 了 always used at the end of sentence when you use 已经。 还 means still，used here to make the sentence more smooth 们 is not necessary here even if you want to look for more than one teacher. I use 个 here because I think you want to find only one teacher, you can delete it if you want.