I got lucky and scored on a large light box at a Goodwill Outlet. I paid 79 cents for it! It's really thick and clunky compared to the modern LED ones, but can't beat 79 cents and it works great.
Before I had that, and what I still do if I want to do something LARGE, is just put a lamp underneath my glass-top art desk. That turns the entire desktop into a light box.
And before I had THAT, I used a window during the daytime, or the TV or computer monitor at night. But this was back when TV & monitor screens were glass tubes; probably not smart to try to trace on a modern LED/LCD screen without putting a piece of plexi or something between pen and screen.