There are a few slow films out there. The only slow color films in current production that I know of are Velvia 50 and 64T (both the Fuji and Kodak versions).
In B&W there is Ilford PanF 50, as well as some slow films from Efke and Bergger (check Freestylephoto.biz).
I've recently shot a roll of Agfachrome ASX II (ISO 50) slide film (medium format), which I found locally. It's not currently being produced. It was very pretty, but it wasn't much different than Provia 100F, which is a phenomenal film.
I'm not sure what the value is of slow film for its own sake, because there are plenty of very high resolution / fine grain ISO 100 films these days. Astia 100F, Provia 100F, Velvia, and Kodak's slide films (like E100G and E100VS) are all made with the modern 'designer-grain' crystal emulsions (T-grain in Kodak's case), which are extremely fine grained and high resolution.
Tech Pan (ISO 25) is gone, but you can find people selling it here and there, but it's got a strange look and requires some care with its extreme contrast. You can try Gigabit film or some orthochromatic copy films, which are available in various places, and have ridiculous lpm resolution.