Bulk film sold by the "large" manufacturers - Kodak, Agfa, Fuji, Ilford ... lord knows how many others ... is the *same* stuff as is loaded into their 35mm cartidges that you buy and use all the time..
Do not confuse it with the re-spooled motion film stock that was (is?) sold by Seattle Film Works. Check the processing information on the box ... If it says "C-41" or, in the case of transparency film, "E-6", it is OK.
There are differences in 35mm film stocks. Motion Picture film is produced and sold, in LARGE quantities, something like 100,000 feet at a time. It is not as closely controlled as far as film speed, color balance, processing requirements, etc., and each batch that is shipped has its own specific information for exposure and processing. Additionally, it *commonly* has an additional layer on the backing, ususally some sort of carbon-black compound (to prevent light from penetrating the flm and bouncing around in the cameras) that MUST be removed as a first step in processing. To attempt to process the stuff in the usual one-hour machines is disastrous... the chemicals will become *hoplessly* contaminated and the machine interiors gunked up to beat hell.
Additionally, the proper chemistry is not the same - again produced and sold in LARGE lots ~ 500 lbs or so.
After each `session', the motion picture studios usually have unused "ends" left over. That is sometimes sold, and re-spooled into film cartridges and sold at *cheap* prices - but not really, when one considers that the people selling the stuff are the only ones that will process it.
It is true that when Oscar Barnack designed the first Leica that he used motion picture film. How many will recall that the proper name for what we call "35mm" is "Double Frame"? Motion picture cameras run the film vertically: Barnack's camera used the film horizontally - therefore, to get a landscape format, two (2) frames were exposed at a time.
I have "bulk loaded" a *bunch*, in the past. Now most of my work is done on 120 roll film, so it is no longer possible (yeah, I know, I could go to 70mm...).
It is not terribly difficult, and it *will* save a LOT of money.
Bulk film, and bulk film loaders, are readily available from Calumet, B&H....