The way I make kits is to first find a container for the kit. Then I rummage through my spare gear filling in the basics. Next I carry the half finish kit and spare cash with me. As I see items I need for the kit I buy it.
I try to budget a small amount each month for toys? aka survival stuff so that way when I see a great deal or remember I need to restock an item in short supply I have the money.
All my kits are small (backpack or smaller) so this system may not work for larger kits. Just my 2 cents <img src="/images/graemlins/grin.gif" alt="" />