The best purchase any coach can make, in my opinion, is a video camera. It doesn't need to be crazy fancy, but HD is nice. I bought this used Canon for less than $200. Along with the camera, I recommend buying a tall tripod of at least 70", and a high capacity memory card of at least 64 GB. I usually have a parent film, and having a tall tripod allows them to bring the camera up conveniently to eye height for most (the less inconvenient it is, the more likely they are to help out). If you are filming in HD, having a large memory card means you don't have to keep clearing out the memory before each game.
The only instructions I give the parents are to stay as zoomed out as possible, and to do their best to follow the ball. Again, convenience is key, and these are simple instructions to follow. Also, some parents tend to try and zoom in as much as possible, rendering the footage useless, as you can't follow the play coherently. Seeing the 3 or 4 players around the play, rather than just the guy on the ball, is what I want out of the footage. Whenever possible, have the camera person stand on elevated ground for an even clearer look at the game. At the 9v9 level, I've found having a parent at ground level still works okay, but at 11v11 it becomes even more important to have the camera elevated.
The other purchase I've found really valuable is a white board. The one I keep at home is about 24"x18". This one is about $25 on Amazon, but I found a cheaper at Rite Aid. I use this to plan training, brain storm exercises or patterns, write down thoughts or sequences while I'm watching games, and more. It's large enough I can draw legibly and fit a lot on it at once, but still small enough to fit somewhat comfortably on my lap. I do keep an 11"x14" one with me for drawing on at training, but I've found a lot more value having a larger one at home. I'm the type of person who needs to write or draw things out to better visualize them, and this is less wasteful, quicker, and less of a hassle than using pen and pencil.
***Honorable Mention: electric air compressor. It gives you no excuses not to have properly inflated balls every session.