Any drugstore or grocery has Ensure or other drinks or I'd learn to make milkshakes and smoothies, etc.

You lose weight by eating less and moving more so you have to gain (unless you have a medical problem) if you eat even a tiny bit more than your normal and move less. Take up reading in your favorite armchair? If you're exercising more than about a half hour a day, you can cut that out; half an hour maintains good health/fitness.