there are a lot of thing that you cannot simulate. Such as HF leakage through ground, near field coupling and other.
If you want to get clean DC it's not easy, and for really quiet DC, you're need to use linear power supply with RF shielding.
Regarding to solutions, you can use 1 MHz switching power supply and ferrite beads with high resistance on HF and high quality low pass filter with high suppression. It can help to reduce HF leakage. Higher frequency for switching power supply helps to make more wide gaps between spurs, it helps to filter them more easily, because your filter may use more flatter slopes. But if your equipment needs very clean DC, it also cannot help. The only way is to eliminate all switching power supplies and use linear regulators.