Rocky Mountain Bonsai Society (RMBS) recognizes that some outstanding bonsai come from collected trees. However, an RMBS member or any bonsai enthusiast should not collect a wild tree without landowner permission (a permit from a land management agency that allows collecting or written permission to collect from a private landowner). Please read the US Forest Service and Bureau of Land Management's policies on collecting trees.
Even with permission, one should never collect a wild tree without knowing how to properly collect and care for that tree so there is a very high likelihood the tree will survive and thrive. If an enthusiast doesn’t have the ability and permission to collect, they should use other sources of material (a nursery for example) or buy from reputable collectors who sell legally collected material.