As far as places I rockhound, greater than 90% of the time I utilize PUBLIC fishing access spots on the Yellowstone river. KatyDidRocks has a great book out on all the places us Montana rockhounds like to go on the Lower Yellowstone River:
https://www.etsy.com/listing/1089523180
While you are there, check out her store! She has some fantastic Montana rocks for purchase.
KatyDidRocks Etsy Store:
https://www.etsy.com/shop/KatyDidRocks?ref=simple-shop-header-name&listing_id=943508179
Also, if you follow Theo Kellison on Youtube, another fantastic Montana Rockhound, he has an Etsy Store with rocks for sale as well:
https://www.etsy.com/shop/SticksandStones406?ref=shop_sugg
Thanks for following and Rock On!