I purchased a bag sealer the same time I bought the bags. Got them off Amazon. 4 mil thickness if I recall correctly. The bags are pretty cool. After you put your food in, add an oxygen absorber and seal it, when the opportunity requires you to open the bag, you don't open the sealed end, you open the opposite end. You can then use whatever portion you need and close the bag with the ziplock seal on the bag. I would recommend using the reserved portion rather soon as it is no longer oxygen free and will spoil if not used within a reasonable time.Do you vacuum seal the mylar bags? What kind of mylar bags do you use? I saw a prep video on youtube where a guy was sealing mylar bags with a vacuum sealer. His method looked like it was ripe for potential failures.
I have seen some Youtube videos of people using an iron and a 2x4 to seal mylar bags but for the additional expense I feel the bag sealer is worth it.