Mylar is very much a US thing. In the UK we call them 'barrier bags'.

They are made up of layers laminated together, one for strength, one to keep out moisture, one to block Oxygen and finished on the inside with a foodsafe layer and so on. I have seen seven layer bags made for a special purpose. The genuine, embossed Foodsaver bags are barrier bags.

Have a look at UK packed bacon, that is a barrier bag, very commonly used. Easily available and a lot cheaper in the UK than Mylar (which is a trade name)

They are also available as 'barrier retort pouches' so they stand up and you may cook in them sealed as well, even in a pressure cooker. This will to a certain extent sterilise the contents and let you keep the meal for a while, bit like a MRE.

This company specilise in vacuum pouches but others are available.
http://www.thevacuumpouch.co.uk/index.php

And
http://www.krehalonuk.co.uk/retortable.php