You can buy 8mm film from the Kodak web site. The film is listed in the Motion Picture Catalog.
Ektachrome 100D reversal film sold in 16mm form with 8mm perforations in a 400 foot roll.
Tri-x reversal film also sold in 16mm form with 8mm perforation in a 400 foot roll.
There is also Kodak Vision colour print film 3383 sold in 16mm form with 8mm perforations in a 2000 foot roll. This film would be used to print from a colour negative film like the Vision 2 or Vision 3 series films.
This is where the 8mm film suppliers get their film. They buy it in 400 foot rolls and cut it to 25 feet spools to sell.