hush hush secret... A few Canon Eos bodies have sprocketless film advance, you can basically bulk load whatever unperforated film and shoot away.

p.s. If you dont mind shooting very slow speed film (think kodalith 6-12 iso), microfilms are available in 35mm format most commonly unperforated, which you can develop for continuous tone. It is probably the cheapest unexpired film you can buy, but you usually have to buy a case. These pop up randomly on ebay as well in 100ft to 1000ft cans. And if its expired, its even cheaper. Since the speed is so slow, there is less of a chance of fog as well.

2 recently completed microfilm auctions:

1200ft for $90

3000ft for $70