I got them from Amazon and sized up to the 9oz since I’ll need bigger sizes eventually anyway. They have a 4 oz and 9oz options. Def recommend getting the blue or pink one so the parts don’t get mixed up with your regular bottle parts. https://www.amazon.com/dp/B07FK464WV/

Oh and the assembly instructions that come with the bottles aren’t super clear so the first time setting up can be hard. I just found a bunch of YouTube videos on how to put them together. https://youtu.be/WxE4G0Xt4l0