Alternatively, you can get the official Space Quest Collection. Just like what you can get at GoG, it gives you all six games for $20, and is playable on modern computers right out of the box. However the original Space Quest in the Collection is a graphical remake, made using VGA graphics so it better matches the later games visually. So it depends whether you want the original game in its original form (go to Good Old Games) or with better graphics (Space Quest Collection).
You can still find physical copies of the Space Quest Collection on sites like Amazon. Steam also offers it for download.