several wonderful herbs for this: i would highly recommend blue vervain (Verbena hastata) or white vervain (Verbena urticifolia) taken internally as a tincture for muscle cramping. they both grow abundantly in the somewhat moist/somewhat dry field edges of missouri, and seem hardy enough to be cultivated elsewhere. some white vervain is nettles-like in its pokiness, so harvest with care.
i would also add an anti-inflammatory, such as wintergreen (Gaultheria procumbens), leaves tinctured, and used topically or in small doses internally. it grew in the u.p. in the understory of mixed white cedar and tamarack forests. i’m not sure of its range or other habitats.
some very common anti-inflammatory herbs over a wide range that would help: chickweed is a great common anti-inflammatory (Stellaria media), and works well either turned into a salve or used fresh & mashed up as a hot compress. plantain (Plantago major or lanceolata) works used the same way as for chickweed. yarrow (Achillea millefolium) is great as an ant-inflammatory & mild pain relief, either mashed & used as a hot compress or as an oil. good for muscle pain in that form also.
if you are somewhere that you have access to devil’s club (Oplopanax horridus), the inner root bark and/or inner stalk bark (thorns carefully removed!) can be turned into a salve that brings GREAT pain relief, reduces inflammation, and helps muscle soreness.
another great herb for this is crampbark, or highbush cranberry (Viburnum trilobum, opulis, and edule). the inner stalk bark can be tinctured (or presumably turned into a salve, which i haven’t tried) and is EXCELLENT for severe muscle cramping, both of smooth muscles (think menstrual cramps) and back/joint/etc cramping. highbush cranberry has a wide range, can run rampant & outgrow other similar sized plants in the mid-atlantic, grows much smaller but still in healthy stands in the sub-arctic & at higher elevations in muskegs of s.e. alaska, and grows tall & lush in the moist, dense forest edges & wetlands of the overall pnw.
i know it’s been years since this request was made, but wanted to add my few cents so folks who don’t have access to arnica can look to other potentially more local plant options. there are a Lot of helpful herbs out there to turn to. this is just what came to mind in a few minutes based on what i have personally worked with.