I'd like to second several items, and then add a few of my own.
1. Wireless thermometer. This is absolutely essential, both for the food and for the smoke enclosure. Some people say you don't need one because you should just be able to tell if things are ready by pressing, touching, bending, tasting, etc. That is all well and good, but you'll avoid a lot of problems if you actually measure what you're doing.
2. Disposable pans. You can get a lifetime supply from Costco for under $10. I find the 10x12 pans fit perfectly in my MES and are just the right size for the typical 8# pork butt.
3.
Q-Matz. These are essential if you want to smoke nuts, or anything else that is not easily supported by the typical "oven shelf" that you find in many smokers. I have an MES 30" and you can buy these pre-cut to fit exactly.
4. Rubbing alcohol. This makes quick work of cutting through the smoke buildup on a smoker window, if you have one. It works well on other smokey parts.
5. Pen and paper. If you are new to smoking, write down everything you do (times, temperatures, things you screw up). Unlike most cooking, BBQ is about as far from science as you can get, and there are so many variables that you are going to have to learn by screwing up a lot. The pen and paper let you keep track of what didn't work -- and what DID work -- so you know next time.
And, since people have mentioned beer ...
Back in 1970 I used to travel from Chicago to San Francisco several times a year. I'd bring Coors to Chicago because, back then, they didn't distribute east of the Rockies. On my return trip I'd bring Stroh's to everyone because they didn't distribute very far outside of Detroit. I haven't had a Stroh's in decades, but if I were doing a smoke today, that's what I'd try to get.