I think what you probably need is what is called a 'dry block calibrator' - basically an insulated, temperature stabilised lump of aluminium into which you can insert multiple thermometers for inter-comparison. They are quite compact & generally fit on a bench top. While the calibrator will keep the block temperature stable, the uncertainty in the stabilised temperature is generally quite large. Hence you should also invest in a 'semi-standard' PRT thermometer and accompanying readout, with individual calibration. This should get you to about 20 to 50 mK calibration uncertainty over your temperature range of interest. Depending on the performance (uniformity of temperature between the thermometer positions, temperature stability over time) of your calibrator, you should be able to maintain 0.1 K overall uncertainty. Thermocouples, unless individually calibrated, cannot meet these uncertainty levels (and even then are subject to drift, requiring frequent recalibration).
Handled with care, platinum resistance thermometers are very stable. While annual recalibration is preferable, because the most likely cause of error is shift in their ice point resistance due to mechanical stress or abuse, regularly measuring and recording the thermometer's resistance at ice point, or preferably at the water triple point, will warn you if something has changed.
Maintaining an ice point bath to within 20 mK is not as easy as it sounds, though, and water triple point cells are rather bulky, fragile, and require training and care in use. If it were my decision, I would use a third-party calibration service on a periodic schedule.
[Edit to add] I should emphasise that the job of the dry-block calibrator is just to keep the temperature of two or more thermometers constant and the same, it isn't itself particularly accurate. For that, you use a calibrated PRT as one of the thermometers being compared.