I haven't gone outside in more than a week now.
Yeah, just legit take Vitamin D supplements. Take a 2,000 IU supplement every day, ideally with your largest meal of the day. (Which should really be breakfast. "Eat breakfast like a King, lunch like a prince, and dinner like a peasant")
Also go for a run outside if you can. Trail running is one of my favorite things in life! If you ever find yourself in Scotland, running through the hills is a life changing experience!