I dont know if you can get a blender and mini fridge in your room but, my favorite homemade weight gainer is
4 tbsps peanut butter
2 scoops whey protein (vanilla)
Cheaper than anything you can get at stores but you will need a mini fridge and blender. Fell free to add any fruit you want or ice depeding on the texture you're going for.
btw it's 1000+ cals
If you want something reach out and grab it.
"What lies behind us and what lies before us are tiny matters compared to what lies with in us." Ralph Waldo Emerson