Metal oxide minerals like tantalite (tantalum) and columbite (niobium) often form placer deposits within basins and drainages. The placer deposits are a secondary ore deposits created by the weathering of tantalite/columbite bearing granite and the transport of heavy sands by water and gravity.
Based on this information, a good approach to prospecting for tantalum / niobium would be collect dark/black sand samples from stream beds within a granite mountain range and to collect samples from riverbeds flowing out of mountain ranges with exposed granite. Larger rivers with steep terrain can carry heavy sediment further away from its source area.
Example: Sawtooth Mountains in Central Idaho are a large granite batholith complex.. Salmon river and its tributaries would excellent areas to look for placer deposits (except of course most of this area is designated as National Forest land.)