Many small producers of film are using up large rolls originally produced for specific technical purposes. Many of these films have very low sensitivity, especially those below 20 ASA.
25ASA was once common as Kodachrome, and a faster 64 ASA version.
Probably the lowest ASA monchrome film in quantity prodcution is Ilford PAN F at 50 ASA.
Recently there have been a number of very low asa films made available, usually in 35mm cartridges. These are made from old film that has been commercially stored so has suffered little to no degradation. Such film was produced for copying higher speed film onto. Espcially movie films. The low asa ensured good grain on the image and as all the processing was automatic the low film speeds were not a problem.
The other use was for document copying for microfiche reading. While text only can use high contrast film, recording images required films more akin to those used in cameras and it is some of these now available for photogrpahic use.
Films available current go down to 3 asa.