If you've got a source of bulk herbs, you can go hog-wild with root-beer-style homebrew! Somewhere between 2 to 4 oz of some combo of the following, in a 5-gallon batch, will get the job done:
Sarsaparilla, wintergreen, licorice root, dandelion root, burdock root; optionally add smaller quantities of cinnamon, star anise, lemon and/or lime zest, fresh ginger root (a couple quarter-sized slices, it can easily overwhelm), and sassafras (people say it's carcinogenic, but the studies based on that are really quite flawed and based on vastly higher doses of safrole than one would ever encounter in a traditional root beer; still, you can find de-safroled sassafras teas out there). You can also add black pepper, sichuan peony root, birch bark, spruce tips, nettles...heck, go wild! Don't be shy, as long as you don't use more than 2-4 oz dried, it's pretty hard to screw it up. I just brewed a root beer porter with sarsaparilla, dandelion, burdock, and licorice root...the unfermented wort was delicious, could use a bit of wintergreen to get it more in the root beer side of things, but it's definitely going to be tasty.
I recommend adding some molasses, maple syrup, brown sugar, and/or raw sugar. Go light on the hops, just enough to add some bitterness and a nice "bite" (I used Willamette). Couldn't tell you much about the malt, as I brew gluten-free, but aim for something between a porter and a brown ale and you should be fine. A bit of raw honey at flameout will help add some residual sweetness, or you can use stevia or lactose.
And remind yourself: if people could do this in the 1800s, how hard could it be?