if you use Colonial Charter, then there are both herb seeds and monks who can make herbs on their own.

Also, make sure that there are enough "old trees" around your herb gatherer, becaue that is where the herbs grow, not in newly planted forests.

Happy "herb-ing" ;)

Another detail about herbs: People only use them if they need to regain some health. If they're unhealthy, that's because of they're diet.
There are 4 food groups in Banished: proteins, veggies, fruits and grains. Health will drop if people don't have a balanced diet.
If they only have stuiff from one or two groups, health will drop faster than they can recover with herbs and you'll have trouble keeping up with the demand.
3 groups + herbs is a rather stable thing, people will use an herb or two per year. Once your people eat fro mthe 4 food groups, they'll stop using herbs once their health is back to 5/5.

So basically to fix your herb problem you have 2 options: producing more by placing the herbalist better / growing herbs with a mod (I think there's one called Herb Seeds somewhere), or improving the diets by getting the right crops and making sure people live near a market so they get a mix of food when they go get food.