>>87102
-Milk. Whole milk has more calories obviously and it's also full of protein and calcium. The paleo and vegan arguments against it are bullshit broscience. If you think you are lactose intolerant you can slowly increase the amount of milk till your body tolerates it, try raw milk, or just use lactose-free milk or take lactase pills.
-Sugar. Some stupid people think sugar is inherently fattening, or otherwise inherently bad for you, but that's wrong. Good source of calories that is easy to get down (tastes great). Throw some in a shake. Or for a post-workout you can use dextrose (corn sugar) if you can get it which will stimulate more muscle growth. Sugar is not bad for you unless your diet is otherwise nutrient deficient. If you are worried about not getting enough nutrients you can use cane sugar or coconut sugar instead of white sugar.
-Fat. Coconut oil and butter are good, or if you need liquid fat so you can mix it, i.e. in a smoothie, you can use MCT oil which stays liquid.
Other than that just get creative. I.e. you can throw a cheap cereal like generic corn flakes in a blender, blend it into a fine powder and throw that in shakes.