Many native wildflowers require special processing or a stratification period in order for the seeds to germinate. I'm not sure what if any preparations bitterroot seed needs, but some resources I found from Washington State (
https://depts.washington.edu/propplnt/Plants/Lewisia%20rediviva.htm) suggest 10 weeks ore more cold/moist stratification before seeds are ready to germinate.
There are a couple
native plant nurseries in western Montana. Centerfornativeplants.com looks like an option, as does Blake Nursery, and there may be others. As the spring season unrolls, you may also keep an eye out at
local farmer's markets and other venues; many state extension programs partner with nurseries to host native plant sales as well. If you choose to buy seed this season, I'd definitely ask whether the seed is ready to germinate, or will need cold/moist stratification.
And if nothing else, you can always collect a little local seed and try preparing it yourself for planting next year.
Best of luck!