I bought them on Amazon and got lucky that they fit, but I can't say they will fit every basket.

The basket I'm using (H24): https://www.amazon.com/gp/product/B092SVTY2T/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

51MM paper filters:
https://www.amazon.com/dp/B0B5DPVDW7/ref=twister_B0B7RDKH56?_encoding=UTF8&psc=1

I use the 51MM filters for the bottom and the 54MM for the top. The 51MM fit both the H24 and H28 baskets in the link above, not sure about the others. They also fit the normcore bottomless stock basket pretty well. They are too big for the stock Breville basket though.