Most building supply companies will have something called 'separation fabric'... or Geotex... or woven polyethylene fabric... they're all the same thing. I order it in 17' wide rolls, 400' long, but it is available in many other sizes.
Note that this is not the same as 'landscape fabric', which is usually paper and/or cloth based.
Separation fabric looks a lot like a winter pool cover or a woven plastic tarp.
You don't need it if you put in a very thick base of gravel, or if you have a very solid subbase under the gravel that is unlikely to mix with the gravel.