There is a recipe in the cooking menu under util called 'vegetable starch'. I accidentally discovered that vegetable paste breaks down into adhesive. Specifically, FIVE adhesive.
The recipe calls for 3 each of mutfruit, corn, and tato. All of these can be grown at your settlement, giving you a permanent supply of each. It may take a while to get the operation spun up (not long -- see below for getting started), but since you can harvest and replant, you should be able to get a massive amount of each, eventually.
The recipe also calls for one purified water. If I'd told you that you could turn one purified water into five adhesive, you'd probably be pretty happy, but it gets better. If you build a water purifier in your settlement, you will occasionally get purified water deposited at the workshop. I built an industrial sized water purifier in the river at Sanctuary, and when I went to check I found 31 purified water in my workshop. Also a good source of health.