Best source of NOS or same quality semiconductors

Any tube to get on the market is no problem for any vintage tube devices while repairing solid state vintage components is hassle. NTE or replacement transistors have different shapes and probably different heat dissipation paramenters and can be absolutely useless in the vintage circuits desinged to handle different shape of transistors.
Which source would you choose if any? So far had been struggling and trying to find ones all of a sudden available via ebay, but sometimes not.
Anyone facing same problems? What is best solution?