99Tc being a product of uranium fission, it is extracted only at reprocessing plants for spent fuel rods, whatever the downstream commercial path is, if any. "Someone" at these plants would know which company sells the material extracted by the plant.
The reprocessing plants are very few: Sellafield, La Hague, Tokaimura, definitely some in Russia, probably in the US and China. India, Pakistan and North Korea have some sort of plant to extract plutonium and tritium from the spent rods and build bombs. By the way, the primary purpose of all these plants (except maybe Tokaimura) was historically to feed bombs and has only diversified a bit over time (make MOX fuel, extract maybe the residual 235U, prepare the long-time storage). Good luck.
First difficulty: if you call La Hague and tell "I want to buy some of your stuff" you can be 100% certain to get trouble by the secret police.
Second difficulty: if the market for 99Tc is too small, the plant won't separate it for you, and even if it's already available, it may be too radioactive because traces of other nuclides with a much shorter half-life pollute it.