Those bags are most likely custom made from companies like the one mentioned or overseas. The bags are the type that food products usually go in but obviously you could put anything in them you like.
The companies who produce these usually have high quantity requirements (3000+ units) but will make almost anything you design. Pricing will vary but even the minimum quantities can be expensive if done locally in the US.
Pricing overseas can be way cheaper but again the minimums might be as 5000-10,000 units.