Quote Originally Posted by battra92
So you can just go into your local film lab and ask for the empties and they'll give them to?
I've only done it once (at a local Walgreens), but they forked them over without any big hassle. (The clerk did ask why I wanted them, but it was clearly personal curiosity and not a store policy sort of question.) I recommend buying something when you do this, just to be polite about taking up the clerk's time with your unusual request.

And of course, cracking the DX code is necessary, especially if you want 25 ISO cartridges (easy convert from a 200)
That depends on your camera(s). Quite a few Web pages detail DX codes, like these:

http://www.geocities.com/Yosemite/2131/dx-code.html
http://www.bythom.com/dxcodes.htm

If you require DX-coded cartridges, ask for them specifically, ideally in the speed you need. (ISO 25 and 50 are going to be pretty rare, of course. My guess is most would be 200 or 400.) When I got my load from Walgreens, about half were from single-use cameras. Those aren't DX coded, although some seemed to be re-used (or at least re-labelled) cartridges, and the stickers could be peeled off to reveal DX codes.

In theory, you should be able to change a DX code by scraping off paint or covering up bare metal. I've not tried this, myself.