Guildline models 7334, 9334, 9336, and 9337 are standard resistors stabilized for use in air at room temperature. These unique standards provide the accuracy levels you require without the need for messy oil baths or long wait times for the instrument to reach steady state.
Available in numerous standard ranges from 1μΩ to 100 GΩ and custom ranges to 10 PΩ.
Model 7334 is also acceptable for use an an AC standard.
Model 9334 is available in direct plug-in models for calibration of most popular high-accuracy MultiMeters.