Freed female slaves or any other women can be wet nurses.

Narrated Um Habiba:

(the wife of the Prophet) I said, "O Allah's Apostle! Will you marry my sister, the daughter of Abu Sufyan." The Prophet ﷺ said, "Do you like that?" I said, "Yes, for I am not your only wife, and the person I like most to share the good with me, is my sister." He said, "That is not lawful for me." I said, "O Allah's Apostle! We have heard that you want to marry Durra, the daughter of Abu Salama." He said, "You mean the daughter of Um Salama?" I said, "Yes." He said, "Even if she were not my step-daughter, she is unlawful for me, for she is my foster niece. Thuwaiba suckled me and Abu Salama. So you should not present to me your daughters and sisters." Narrated 'Ursa: Thuwaiba had been a slave girl whom Abu Lahab had emancipated.

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي عُرْوَةُ، أَنَّ زَيْنَبَ ابْنَةَ أَبِي سَلَمَةَ، أَخْبَرَتْهُ أَنَّ أُمَّ حَبِيبَةَ زَوْجَ النَّبِيِّ ﷺ قَالَتْ قُلْتُ يَا رَسُولَ اللَّهِ انْكِحْ أُخْتِي ابْنَةَ أَبِي سُفْيَانَ. قَالَ " وَتُحِبِّينَ ذَلِكَ ". قُلْتُ نَعَمْ لَسْتُ لَكَ بِمُخْلِيَةٍ، وَأَحَبُّ مَنْ شَارَكَنِي فِي الْخَيْرِ أُخْتِي. فَقَالَ " إِنَّ ذَلِكَ لاَ يَحِلُّ لِي ". فَقُلْتُ يَا رَسُولَ اللَّهِ فَوَاللَّهِ إِنَّا نَتَحَدَّثُ أَنَّكَ تُرِيدُ أَنْ تَنْكِحَ دُرَّةَ ابْنَةَ أَبِي سَلَمَةَ. فَقَالَ " ابْنَةَ أُمِّ سَلَمَةَ ". فَقُلْتُ نَعَمْ. قَالَ "فَوَاللَّهِ لَوْ لَمْ تَكُنْ رَبِيبَتِي فِي حَجْرِي مَا حَلَّتْ لِي، إِنَّهَا ابْنَةُ أَخِي مِنَ الرَّضَاعَةِ، أَرْضَعَتْنِي وَأَبَا سَلَمَةَ ثُوَيْبَةُ، فَلاَ تَعْرِضْنَ عَلَىَّ بَنَاتِكُنَّ وَلاَ أَخَوَاتِكُنَّ ". وَقَالَ شُعَيْبٌ عَنِ الزُّهْرِيِّ قَالَ عُرْوَةُ ثُوَيْبَةُ أَعْتَقَهَا أَبُو لَهَبٍ.