I've never produced a stock with only one by-product from the animal except maybe my beef stock. For that I don't like the collagen in the stock and try to use only femurs with no knuckles. The amount of time your cooking, 4 hours is good but I'll sometimes go as long as 6.
That said......try and locate some backs and necks. Unfortunately in our area, there aren't too many grocery stores (if any) that break down their own poultry these days. However, you could probably find a small butcher shop.
As far as cost, if it's a local grocery and they don't already have them packaged, they will probably give you the parts or sell them at a greatly reduced price. The butcher shop probably won't give them away but should sell them at a fair market value. Mostly because this represents part of their cost of goods purchased. Anyhow, from the sound of things, you're looking for that gelatin like consistency when cooled and the necks for sure will contribute with that. Roasting the parts also helps with flavor. Although this is not classic, it's not unacceptable either. The best way is to coat everything with a little oil, including the vegetables mentioned and roast in a high heat oven. The stock will have a darker complexion because of the color applied to the base products but it does enhance flavor a great deal.
Just remember don't create a vigorous boil in any situation when steeping the bones. There should be just a slight bubble in the pot. If it's boiled, your stock will be cloudy. Herbs, as mentioned will help but if you don't put them in a sachet or bind tgem with twine, they can fall apart. You'll definitely need to strain it but if you want a clean stock, herbs or not, remember to strain your stock through some cheese cloth at the end. Make certain you soak the cloth in water and wring it out. This will keep it from absorbing too much stock.