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.