I bought mine from topmylar.com
They sell a wide range of bags in different sizes and weights. I use the 7 mil metallized bags. The metal impedes the permeability of the Mylar, and the bags are a laminate, so 7 mil bags are 7 metallized layers. Heat sealed, that's a solid barrier. If you're going to keep tobacco in square and rectangular tins long term, as in 7+ years and way beyond, this is a way to diminish the risk of loss due to seal failure.
Or you can just jar everything as soon as you buy it, and let the aging process take place.
And before someone posts a link to Greg Pease's FAQ on aging, be aware that he's changed his mind about aging tobacco in the tin, and wrote a letter to BriarBlues youtube channel to that effect. Too many disappointments with his own cellar.
You don't need any fancy equipment to seal the bags. The supplier advised me to buy a hair salon type flat iron to perform the heat seal. I picked up a Kipozi Pro Nano Platinum flat iron on Amazon that works like a charm. I can set the temperature so that I can get the best seal.