When you're running npm install in the project's root, it installs all of the npm dependencies into the project's node_modules directory.
If you take a look at the project's node_modules directory, you should see a directory called http-server, which holds the http-server package, and a .bin folder, which holds the executable binaries from the installed dependencies. The .bin directory should have the http-server binary (or a link to it).
So in your case, you should be able to start the http-server by running the following from your project's root directory (instead of npm start):
./node_modules/.bin/http-server -a localhost -p 8000 -c-1
This should have the same effect as running npm start.
If you're running a Bash shell, you can simplify this by adding the ./node_modules/.bin folder to your $PATH environment variable:
export PATH=./node_modules/.bin:$PATH
This will put this folder on your path, and you should be able to simply run
http-server -a localhost -p 8000 -c-1