A lot of people may be aware of this and do it but I thought I'd post it, there are always new owners or new challenges it might help.
Wait till chicken thighs are on sale as cheap as you think they ever will be. (sometimes on the last sell by date they are 40% off here).
Take a couple, place in a pot of cold water covered by a couple of inches of water, heat on stove to a gentle boil and boil the crap out of them for a little bit, say around 20 mins. If you have poultry or kitchen shears cut the bones in half so the marrow extracts (watch the splash, tongs help). I kind of pull things apart to make sure every bit gets cooked/extracted.
Lower the heat to a low simmer, Let go another 30 minutes or until concentrated enough to be a good broth and not too watery.
Put a bowl in the sink, place whatever kind of fine mesh strainer you have over it, or a collander with some cheesecloth and pour the contents through the mesh.
Voila, a nice flavorful broth with no spices etc. like in the store bought.
Cool, freeze into ice cubes, store in ziplock bag or container.
At this point there isn't much nutrition left in the mean but you can pick out the big chunks and they like it anyway
When you want give a little liquid treat, or make some food more interesting, or get liquids into a sick cat,
gently microwave a couple of cubes to melt, let cool to just warm and put in a bowl.
Your cat should like it.