I got mine on eBay, a few at a time. The prices range from good to ridiculous. If you have patience, you'll eventually get all you need:
https://www.ebay.com/itm/194092852008?hash=item2d30d5e328:g:7eUAAOSwVuRgdJzh https://www.ebay.com/itm/284276061721?hash=item42302c7619:g:fPkAAOSwDbJgWlqA
Four is enough for most stations, although I have made eight for my '70s station. I scratchbuild a little at a time, as I feel like it. All the items in the picture are scratch.
Good luck with your station, I hope this helps!