Beans are the classic "filler" but I find them to be too lumpy/heavy. The rice/lentils just seem to feel better in the hand. But experiment with what you have in the kitchen... you might find the perfect mix!
Hopefully someone will have a fav online supplier. I'm lucky that our local co-op has a great dried herb section. I'll confess to "cheating" a bit on some sachets I made once... I had gathered some herbs from my garden but didn't have enough to fill the bags I'd made. So I used tea bags.
I think Frontier or another online herb shop would be a better choice, but if you ever find yourself needing a last minute "smelly" try the tea bag approach.