What form? Maybe substrate.
From MatWeb: AN 180 Aluminum Nitride Ceramic Substrate, 99% Purity
Maximum dimensions 5.5 x 3.6 in. Thickness 0.12-0.60 in. Surface roughness 0.3 µm as fired; 0.075 µm lapped; 0.025 µm polished. Supplier: MarkeTech International, Inc. Phone toll free at 877 452 4910.
Others include AREMCO AREMCOLOX™ Machinable and Fired Ceramics 502-1800 Aluminum Nitride. Hot-pressed Aluminum nitride. Ultra high thermal conductivity and dielectric strength. Readily machined. Plates from 1/4" to 1" thick, custom rod available upon request. AREMCO.
Ceralloy 137, Aluminum Nitride, AlN
This thermally conductive and electrically insulating ceramic is recommended for use in travelling wave tubes, laser systems and as a substrate material for electronic circuitry where good heat transfer is required. Data provided by the manufacturer, Ceradyne Advanced Ceramics.