I started by looking at he kit, not buying them, looking at things I had and how the things I had could to the things the stuff in the kits were intended to do, and then also learning about some substitutes.

For example, an off the shelf 72 hour kit will probably have a rain poncho made from PVC. However, a plastic garbage bag is in many cases more durable. You might be able to scrounge strong garbage bags for free. So, you can make a few of them part of your kit(s). In the end, you'd probably prefer a military spec poncho, with quilted liners, maybe setup to function as a sleeping bag, but add them as budget allows, and maybe you can find options that work better or are cheaper, and fill us in.

For food, try to adjust what you eat toward what can be stored and build things up as you can, rotatig stocks.