Galbanum comes from Ferula gummosa and its relatives, tall umbellifers of the Iranian plateau and Central Asia, tapped by cutting the stem near the root and collecting the milky exudate as it hardens. The same genus produces asafoetida, which is the reason the smell is familiar to anyone who cooks Indian food, and silphium, the Roman contraceptive and seasoning that was harvested to extinction and is the ancient world's clearest case of a plant used up.
The temple incense is the other reason the genus matters here. The Talmud names the house of Avtinas as the family who compounded the ketoret, records that they refused to teach the method to anyone else, and reports that they were criticised for it and defended themselves on the ground that the secret would be misused. The tractate also preserves the detail that no bride of the family ever wore perfume, so that no one could say they took the temple's incense home.
Whether the eleven-ingredient recipe as transmitted can be reconstructed is doubtful, since onycha is unidentified and several other names are contested, and the question is a live one for anyone attempting it.
Ferula resins are still collected across Iran, Afghanistan and Central Asia by essentially the ancient method, and the trade is small, seasonal and largely unrecorded.