Doing hill sprints without hills is, well, quite hard. Not impossible, though. There's the obvious choice of treadmill intervals with a significant incline, but you could also try to define 'hill' a bit more loosely.
Photo by Don Shetterly on Unsplash
I do 8*400 meter hill sprints every week and I use a bicycle tunnel near my house for that. The tunnel is 900 meters long, but the lowest point is not in the middle. From this lowest point to the most nearby exit is exactly 400 meters, so that's perfect. I know people who use the ramp of a parking garage/ I guess these are shorter, but steeper.