As long as your teas/herbs are well dried (no bendy/softness to them) when they go into the tins, then it should be fine without a dehydration sac. Keeping dry herbs in a non breathable container is preferred, in order to prevent them from re-hydrating so well done on that choice.
If you are concerned that there is still moisture in the herbs before putting them in the tins, perhaps give them a bit of time in the oven with the light on, or a dehydrator first.
I don’t personally know of any suppliers of natural dehydrating sacs, but you could potentially make ones yourself with rice in a little sewn sachet/pouch that you make or buy.
I hope this helps a little, best of luck with your herb storage!