You could just use the input noise of an amplifier. It may not be flat and you will need enough gain to get from ktB to the power level you want. The amplifiers will need to have a good frequency response if you need wideband noise, and the input screening needs to be perfect.
A noise diode would be the best way to get a reasonably flat power density. Some diodes give around 30dB over ktb. If you don't have a noise diode a zener works reasonably well at low frequencies, and a reverse biased base emitter junction of a microwave bipolar is OK to a few GHz. Not too flat though.
Peter