What's the most cost effective way to make chicken bone broth?

Formulas for stock are somewhat variable, but a common case is to use 3 kg of bones (and half a kg of mirepoix, which is a vegetable mix used for taste) with 4-5 l water, which after cooking down yields 3 l of stock, or just a little bit more than that. I couldn't find an especially good figure for the bone:meat ratio of chickens, but many Internet sites seem to agree on a 30:70 figure (I hope they didn't copy it from each other without fact checking). So, you are looking at either buying 1 pound of bones, or one 3-pound chicken per day.

In your place, I would prefer to use bones, not whole chickens, because of the logistics involved. With chickens, you'd need lots of freezer space, and then will end up with 2 lb of chicken meat daily. The work of removing the bones, especially if your recipe requires you to do so before boiling, will also cost a lot of time. But I don't know in what circumstances you live, maybe these points are not a problem for you.

If you decide to use bones, the first place to look for them would be a local meat seller. I have heard that in the US and Canada, bones are seen as a waste product and sold cheaply. If you cannot find a convenient place which sells them, you might have to ask a butchering company. They will have many bones, but are unlikely to bother to trade in small amounts, so while you are likely to get a good price, you might have to deal with a bulk delivery (and the uncooked bones will need freezing if they are to last for more than a few days).

A catering business or a restaurant might be another place to ask, as showbiz suggested, but you should be aware that fast food chains get their halfway prepared ingredients delivered from a central factory, and high-end restaurants use the bones for cooking stock for their own needs.

If I were you I'd contact some food/catering companies nearby. I'm sure some of them just use the meat and throw the bones away. So it's possible that they may give the bones to you for free or for a real small amount.