Wal-Board Tools 4 in. x 3.5 in. Inside Corner Tool Trowel 082-032-HD - The Home Depot
A Pair Drywall Corner Tool,Inner Outer Wall Corner Trowel Plastering Tool Perfect 90 Degree Right Angle when Mudding Drywall Plastering Around the Groove - Amazon.com
Drywall Corner Tool, Heavy-Duty Yellow Plastic | True Value