For SARS1 they detected the virus from in Himalayan palm civets at a market in Guangdong. "All the animal isolates retain a 29-nucleotide sequence that is not found in most human isolates." - which means the virus was not from human to civet as opposed to human -> civet
https://www.science.org/doi/10.1126/science.1087139
For MERS(outbreak 2012) they linked the intermediate species to be camels https://www.science.org/content/article/new-middle-eastern-v...
One thing you'll notice when reading the studies of these two viruses they both had multiple spill overs since the virus was circulating in an intermediate species.
When you hear "It took years to find the source", what they are talking about is the original bat source that the virus originated before it infected the intermediate hosts. These statements are disinformation since when people are talking about "finding the intermediate host" they are speaking of the animal the that triggered the spillover to humans which is much easier to trace and can be found quite quickly.